AbstractPrior research into the cost of trading on the Australian Stock Exchange has identified brokerage fees and the bid‐ask spread as significant elements of total transaction costs. While an enormous volume of research has examined the determinants of spreads in US markets, no work has so far addressed the issue for the Australian market‐place. Given the importance of spreads as a transaction cost, this work redresses this imbalance and at the same time provides evidence on whether alternative market structures underlying different exchanges give rise to differences in the determinants of spreads. Using prior US research as our benchmark, our results suggest that notwithstanding microstructure differences between the Australian and US exchanges, there are three fundamental determinants of spreads that transcend differences in the market‐places. These are the level of trading activity, price volatility and stock price levels. Together these three factors account for up to 94% of the total cross‐sectional variation in percentage bid ask spreads on