Nowadays, the search for new materials with a sustainable character to reduce the production of residues is under continuous research. In this sense, fully biodegradable composites based on polyhydroxybutyrate and different pretreated fibres coming from barley straw have been fabricated, and their resistance to environmental controlled conditions have been characterized. The materials were already compounded in a kinetic mixer and injection-moulded as specimens for tensile assay to be aged in a Xenotest chamber so as to simulate environmental conditioning. The samples, after accelerated aging, were characterized thus: mechanical characterization (tensile assay), water uptake (immersion and contact angle), and surface observation (optical and SEM microscopy). The incorporation of the fibres helps the composite to keep its structure for a longer time. On the other hand, the presence of the fibres increases the water uptake capacity to allow water permeation in the composite, which allows final degradation, characterised by a significant drop in properties after one month of exposure to simulated environmental conditions.