Second order nonlinear optical properties of poled thin films of a guest-host system consisting in a charge transfer chromophore dissolved in poly bis(phenol A carbonate) matrix were studied by optical second harmonic generation and as function of chromophore content. The chromophores were oriented by the corona poling technique. The optical microscopy observations evidence a new mechanism of relaxation, which arises from the accelerated chromophore aggregation due to the heating which facilitates their diffusion. (c) 2007 Elsevier B.V. All rights reserved.