Three common structures
Complete set
Buy every mutually exclusive outcome when total executable cost plus fees is below the fixed payout.
YES + NO
Buy complementary sides when combined all-in cost is below $1 and both resolve under identical rules.
Cross-venue
Take opposite positions on two venues when the locked payout exceeds cost—but only after comparing their contracts line by line.
The minimum calculation
Example: YES is offered at 47¢ and the economically complementary NO is offered at 51¢. The headline cost is 98¢ for a potential $1 payout. The apparent edge is 2¢, but it disappears if fees and slippage exceed 2¢—or if the contracts can resolve differently.
The contract-equivalence test
- Do both contracts use the same event, deadline and time zone?
- Are postponements, recounts, substitutions and cancellations handled identically?
- Do they name the same resolution source and correction window?
- Can either venue void, settle early or use a discretionary committee?
- Are all outcomes truly mutually exclusive and collectively exhaustive?
If any answer is unclear, the trade is basis risk—not a locked arbitrage.
Execution and capital risk
Visible prices are not reserved for you. One order may fill while the hedge moves, leaving directional exposure. Use executable depth, conservative size and limit orders where appropriate. Also plan for funds to remain locked until both venues settle and permit withdrawal.
99market does not execute arbitrage or assert that displayed opportunities are risk-free. Use the APR calculator for time-adjusted comparisons and read the risk disclosure.
Frequently asked questions
Is prediction market arbitrage risk-free?
No. A quoted spread can disappear, one leg may not fill, rules may differ, a venue can halt trading, and settlement or withdrawal delays can change the result.
What is complete-set arbitrage?
For mutually exclusive outcomes that collectively pay exactly $1, a complete set may be underpriced when every outcome can be bought for less than $1 after all costs. The rules must truly make the outcomes exhaustive and exclusive.
Why can the same event have different prices?
Venues can have different participants, liquidity, fees, eligibility, settlement sources and wording. A price gap may compensate for a real contract difference rather than represent free profit.