S&P 500 (SPX) Up or Down on July 31?

Platform comparison

Market context

The cash S&P 500 finished **higher on the latest prior trading day**, ending Thursday at **7,437.63** after a 1.7% rebound, and then rose again on Friday to **7,489.72** by the close reported in market coverage, which is the key setup for this contract’s “Up” resolution if that closes as the official final print.[2][1] With the market implying **100% YES**, the pricing is already treating an up close as a certainty rather than a probability, so any comparison with sportsbook-style odds is effectively moot unless a venue posts a rare alternative line.

That reading fits the broader tape: the index had been under pressure over the past month, with MarketWatch showing a **6.27% one-month decline** and **5.11% year-to-date loss** before the late-July rebound.[3] Historically, late-month S&P 500 direction is driven more by event risk than by calendar effects alone, and a 1.7% one-day gain after a sharp sell-off shows how quickly sentiment can turn even inside a weak monthly trend.[2][3] In that sense, the current market price reflects a strong consensus that the rebound on 31 July was enough to leave the index above the prior close.

The main catalysts for a trader to watch are the day’s post-close market reports, the final official S&P 500 close used for settlement, and any late-breaking macro or earnings surprises that could alter the closing level before 4 p.m. New York time. Recent reporting pointed to investors reacting to Federal Reserve policy being left unchanged and to earnings-season rotation into tech and consumer discretionary names, both of which helped the index recover.[2] Reuters also noted that futures had been sensitive to earnings and geopolitical headlines in July, reinforcing that this contract is mostly a closing-price question rather than a broad macro forecast.[16]

Sources: 1 · 2 · 3 · 4 · 5

Methodology

This page reviews S&P 500 (SPX) Up or Down on July 31? across five venues. The live probability is the Polymarket mid-price, sourced directly from the on-chain Polygon order book; the comparison columns benchmark each venue on fee structure, KYC, settlement currency and payment rails. Every CTA routes to Best Prediction Markets, which mirrors the Polymarket order book at 0% fees.

Resolution & payout

Polymarket-based markets settle through the UMA Optimistic Oracle on Polygon. A proposer submits the outcome, a two-hour challenge window opens, and unchallenged proposals finalise the resolution. Payouts settle automatically in USDC the moment the result is final — no bookmaker, no delay.

Kalshi-based markets settle in USD via the CFTC-regulated clearinghouse. Betfair Exchange settles in GBP/EUR net of commission. Manifold is play-money and does not pay out real funds.

FAQ

Where can I trade this market with the lowest fees?

Polymarket is geo-blocked in the US/UK/EU. The easiest 0%-fee broker into the same order book is Best Prediction Markets. Kalshi charges up to 7% per trade; Betfair Exchange takes 2-5% commission on net winnings.

Is this market available outside the US?

Polymarket itself is geo-blocked in the US/UK/EU. Always check the legal status of prediction markets in your jurisdiction before trading.

What's the difference between YES and NO shares?

A YES share pays $1.00 if the event happens, $0 otherwise. A NO share pays $1.00 if the event doesn't happen. The market price between 0¢ and 100¢ is the implied probability.

What does Polymarket cost to trade?

Polymarket itself charges 0% — the only cost is the Polygon network fee, typically under $0.01 per transaction. Off-chain venues like Kalshi or Betfair charge 2-7% commission.

How reliable are the quoted odds?

The YES/NO percentages are the live mid-prices of the Polymarket order book. On deep markets they move every few seconds; on thinner ones you'll see short plateaus.