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Biodegradable plastics can reach full degradation when disposed of appropriately and thus alleviate plastic pollution caused by conventional plastics. However, additives can be released into the environment during degradation and the fate of these additives can be affected by the degradation process. Here, we characterized TiO 2 particles released from a biodegradable plastic mulch during composting and studied the transport of the mulch-released TiO 2 particles in inert sand and agricultural soil columns under unsaturated flow conditions. TiO 2 particles (238 nm major axis and 154 nm minor axis) were released from the biodegradable plastic mulch in both single-particle and cluster forms. The mulch-released TiO 2 particles were fully retained in unsaturated soil columns due to attachment onto the solid-water interface and straining. However, in unsaturated sand columns, the mulch-released TiO 2 particles were highly mobile. A comparison with the pristine TiO 2 revealed that the mobility of the mulch-released TiO 2 particles was enhanced by humic acid present in the compost residues, which blocked attachment sites and imposed steric repulsion. This study demonstrates that TiO 2 particles can be released during composting of biodegradable plastics and the transport potential of the plastic-released TiO 2 particles in the terrestrial environment can be enhanced by compost residues.