Critical micelle concentrations (cmc’s) were measured for a range of block copolyelectrolytes based on styrene (the insoluble block) and sodium acrylate. The lengths of the styrene blocks ranged from 6 to 110, while those of the polyelectrolyte ranged from ca. 300 to ca. 1400. The fluorescent probe technique was utilized with pyrene as a probe molecule, and the data were treated by five different methods. cmc results were interpolated for a constant polyelectrolyte block length of 1000 units. It was found that changing the insoluble block length from 6 to 110 lowered the cmc from 1.6 X 10(-5) to 5 X 10(-8) M. By contrast, changing the soluble block length from 300 to 1400 typically changed the cmc values by less than a factor of 2. For very short PS block lengths, the cmc decreased very rapidly with increasing length of the insoluble block. By contrast, for higher block lengths (above 12) the drop in the cmc was much more gradual. The results are compared with those of previous studies of block copolymer micelles and theories of block copolymer micellization. The following publication treats the results within a new theoretical framework.