Abstract Simultaneous measurements of reactant conversion and bubble characteristics were made for a two-dimensional, bubbling, fluidised bed of catalyst. A porous plate and a sieve plate distributor were used. The results obtained suggest that the effect of distributor type on conversion is largely a consequence of the effect of distributor type on initial bubble size. For the present results, the two-phase theory is shown to provide a reasonable correlation between these effects, provided that initial mal-distribution of gas is avoided. Comparison of the predictions of a simple bubble model with experiment indicates that bubble/particulate phase gas exchange is enhanced at high, bubble phase, flow rates. At low flow rates the gas exchange predictions of Hovmand 8 and Walker 11 are adequate. At the high flow rates typical of many industrial beds, the simple gas exchange prediction of Davidson and Harrison 14 leads to the best prediction of conversion. The results obtained confirm the validity of the Fakhimi formula 12,13 for the design of multi-orifice distributors although such design appears to be conservative.