The effect of urea, N-monomethylurea, and N,N'-dimethylurea on the micellization of the nonionic ethoxylated tenside pentaethyleneglycolmonohexyl ether (C6E5) was investigated through intradiffusion coefficient measurements (Pulsed-gradient spin-echo-NMR method) at 25 degrees C. The presence of the additive causes, in all cases, an almost linear increase in the critical micellar composition (cmc), while a decrease in the aggregation number occurs. Furthermore, both the monomer-monomer and micelle-micelle interactions increase. The analysis of the intradiffusion coefficients shows the presence of additive molecules in the hydration shell of the tenside, in both its monomeric and micellar forms. A comparison with the results obtained for the water-pentaethyleneglycol (PEG5)-urea or alkylurea systems suggests that the additive molecules are mainly positioned near the tenside ethoxylic chain. The number of alkylurea molecules in the solvation shell of each C6E5 and PEGS molecule was computed and commented. The experimental cmc values in watcr-C6E5-urea mixtures are in quite good agreement with those predicted through a molecular thermodynamic model.