In stockpiles exposed to high relative humidity (RH) and rainfall, woody biomass pellets lose structural integrity, often assumed to be due to the uptake of moisture from the environment. In this study three different types of biomass pellet were artificially degraded in a laboratory environment under controlled exposure to RH (10% and 90% RH) and temperature (range of 10 to 30 degrees C). White, torrefied and steam-exploded wood pellets were investigated. Daily shear tests were conducted with durability and moisture content measured. The exposure of all three pellet types to high RH coupled with elevated temperatures caused a substantial decrease of shear modulus with values of 50% to 92% decrease compared to fresh pellets after 4 days of exposure. The steam exploded pellets saw the lowest drop in mechanical durability (5%) but saw the largest decrease in shear modulus, whilst the white wood pellets disintegrated in situ after 4 days. In contrast storage at 10% RH did not cause any observable degradation, with mechanical behaviour of steam exploded and torrefied pellets showing an improvement. This paper presents both testing methodology as well as clear indication of the behaviour of three woody biomass pellets on exposure to high relative humidity. (C) 2016 The Authors. Published by Elsevier B.V.