Mass-transport enhancement by rising single and multiple bubble streams was monitored on a vertical electrode incorporating a mosaic of 100 x 100 mum sensing elements. Limiting current measurements of the reduction of ferric to ferrous sulfate revealed the presence of two distinct mechanisms : surface renewal is effective within a swath of five bubble diameters, while macroscopic laminar flows induced by rising bubbles contribute weaker enhancement over a wider path. Enhancement caused by a sheath of multiple side-by-side bubble streams can be well represented by the linear superposition of the effect of single streams. Generally, at equivalent volumetric rates, small bubbles are more effective than larger ones in reducing mass-transfer resistance to the underlying surface.