The behavior of binary composites (BC) containing polyvinyl alcohol (PVA-1799) and various amounts of microcellulose (MC) in water and biochemical media (soil, compost, water-silt mixture) is studied. The results of the experiment emphasize the complex nature of the biodegradation of the studied composites under natural conditions. Introduction of microcellulose into the polymer matrix of PVA about 60 vol.% is accompanied by the formation of a porous structure that promotes the penetration of water and components of biochemical media into the volume of the material. At the same time, the uniform distribution of the MC fibers creates the effect of a reinforcing filler and allows you to maintain strength even with prolonged soaking of the BC. The achievement of "zero" strength at exposure in water is observed after 2 days, 14 days, 2 months. for BC with a content of MC 0, 80-60, 40-10 vol.% , respectively. After 6 months of incubation in the soil environment, the index of destruction of composites was 0.89, 0.87, 0.95, 0.96 with a degree of filling with microcellulose of 10, 20,40, 80 vol.% , respectively, while there was a lack of fragmentation of the samples. A method of computer colorometry in the dynamics of biodegradation of polysaccharide-filled thermoplastics is proposed to assess the degree of penetration of components of biochemical media into the volume of the material.