Microencapsulation of fish oil aims to protect polyunsaturated fatty acids against oxidation by embedding oil droplets in a solid matrix. To promote autoxidation, oxygen has to penetrate towards the encapsulated oil. The aim of the study is to quantify the impact of the powder particle size on oxidation. An O/W emulsion (1.6% soy protein; 30.4% maltodextrin; 8% fish oil) was spray dried and the obtained powder sieved into four size fractions (<50 mu m, 50-80 mu m, 80-125 mu m, >125 mu m). Particle fractions and bulk powder were stored for 151 days at 25 degrees C. The hydroperoxide concentration and Anisidine Value in the total- and encapsulated oil were measured. Small particles oxidized significantly faster than large ones. The hydroperoxide concentration quadratically increased with the total interfacial area. Consequently, it was inversely proportional to radius squared, indicating that oxidation was diffusion limited. The experimentally obtained relation between particles size and oxidation products is in accordance with diffusion theory. (C) 2020 Elsevier B.V. All rights reserved.