We investigate nonlinear refraction of chloroaluminum phthalocyanine dissolved in ethanol using the Z-scan technique with 532-nm 19-ps laser pulses, separated by various time intervals with respect to the solution's mass diffusion time constant. As a result, we verify that a negative lensing effect is induced by the outward solute migration, in agreement with our theoretical inference. (C) 2009 Published by Elsevier B. V.