The rotational diffusion of two nonpolar laser dyes (Exalite392A and Exalite398) is studied as function of viscosity in alkanes and alcohols by steady state florescence depolarization technique. Results are analyzed with the extensively employed SED hydrodynamic theory. Contrary to expectations, sub-slip behaviour has been observed for the dye Exalite398, which is the largest molecule studied so far in literature. Both the probes are found to rotate much faster in alcohols than in alkanes of equal viscosity. Applicability of quasi-hydrodynamic models of GW and DKS is also discussed. (c) 2006 Elsevier B.V. All rights reserved.