In previous studies moisture diffusion coefficients for transformer pressboard insulation impregnated with mineral oil have been proposed. In this paper diffusion coefficients are proposed for transformer pressboard insulation impregnated with different natural esters. The coefficients were validated with experimental data and compared with those of mineral oil. In the determination of the moisture coefficients drying experiments at different temperatures were performed on pressboard samples of several thicknesses. An optimization process based on particle swarm and a finite element model were used to derive the coefficients from experimental data.