This study investigated the diffusion of peanut oil and counter-diffusion of cocoa butter when cylindrical tempered or untempered samples, composed of cocoa butter with or without addition of cocoa powder, were suspended in peanut oil and stored at 23 degrees C. Oil migration kinetics was monitored using a novel pipette technique while diffusion and counter-diffusion were measured using gas chromatography. Tempering significantly lowered (p < 0.05) the peanut oil diffusion in the samples but did not influence the counter-diffusion of cocoa butter in peanut oil. Addition of cocoa powder at a volume fraction of 0.45 significantly increased (p < 0.05) peanut oil diffusion and decreased counter-diffusion. Due to the ingress of peanut oil, the solid fat content of tempered and untempered cocoa butter samples dropped from 75% to 59% after 7 weeks and resulted in corresponding dimensional changes in the samples which were also measured using image analysis. Diffusion, counter-diffusion and lipid phase changes were combined to develop a mass balance model to predict the volume changes occurring during oil migration. (C) 2010 Elsevier Ltd. All rights reserved.