By authorising the international exchange of allowances and reduction credits for greenhouse gas emissions, the Kyoto Protocol ＇flexibility＇ mechanisms will spawn a global market for greenhouse gas emission offsets. If conditions for ＇supplementarity＇ - the extent to which countries may use international mechanisms relative to domestic efforts - remain relaxed, market access and liquidity is expected to be high. This scoping paper aims to broadly assess market size and value. By comparing future allowances with varied economic growth-sensitive emissions projections for countries with emissions limitation commitments under the Protocol, we estimate the annual size of this global offset market to be similar to 850-1500 MtC/yr during the 2008-2012 commitment period. Further we find no more than 1/3 of market demand can be satisfied through ＇hot air＇ trading, ie, the exchange of certain countries＇ excess allowances. Variable costs for supply-side options in this global market, and uncertainty on how future supplementarity policies might limit free-market demand-side allocations, render a precise estimation of market value difficult. However, considering two supplementarity cases and cost data consistent with recent studies, we infer total market value to be similar to 24-37 $b/yr during the commitment period, of which 9-17 $b/yr would take place through flexible mechanism transactions.