Diffusion of guest species in particles of host material in composite electrodes is investigated for potential step and EIS methods taking particle size distribution into consideration. Theoretical expressions are derived for the output response to a potential step and the diffusion impedance considering linear diffusion in particles. Numerical calculations are used to demonstrate the effects of particle size distribution. Generalisation is possible considering other particle and/or diffusion geometries. Previous results in electrochemical literature are discussed in the light of the theoretical derivations proposed in this paper.