An aqueous micellar radical copolymerization technique has been used to prepare water-soluble copolymers of acrylic acid/styrene, acrylamide/styrene and acrylic acid/ethylphenylacrylamide. In this process, the hydrophobic monomer (1-2 mol% based on the monomer feed) is dispersed within surfactant micelles while the water-soluble monomer is dissolved in the aqueous medium. This synthesis leads to block copolymers in which the number and length of the hydrophobic blocks vary with the degree of conversion. Kinetic studies have shown that the compositional heterogeneity results from the balance between the values of the reactivity ratios of the monomer pairs and the micellar effect inherent to the technique. The latter always leads to an enhanced apparent reactivity for the hydrophobic monomer. It is shown that an optimization of the process can be achieved which eventually corrects for the drift in the average copolymer composition with conversion. The thickening ability of the hydrophobically modified water-soluble copolymers is directly affected by the extent of their compositional heterogeneity. Analysis of the data provides some criteria for selecting appropriate systems with good controllable theological properties.