Particulate organic matter (POM) is a key component of aquatic system functioning. It mainly comprises i) autochthonous macro and microphytes and ii) more refractory allochthonous terrestrial organic material. Each differs by origin and role in aquatic biogeochemical and ecological processes and is challenging to differentiate. POM origin and composition are usually quantified with C and N elemental and isotopic ratios and by applying Bayesian mixing models. However, these tools may suffer of a lack of source discrimination in rivers; hence new tracers of POM origin are needed.   Terrestrial organic matter ²H:¹H ratios are controlled by meteoric water but differ significantly between land and aquatic-based primary production. We hypothesized that this ratio of the non-exchangeable H of POM could independently quantify POM composition over three years in the Loire River, France, at the river-estuary interface. Previous work using C and N elemental and isotopic data suggested that POM comprises terrestrial matter and phytoplankton. The POM non-exchangeable ²H:¹H values revealed distinctive signatures: the terrestrial POM component had higher values but was constant over time (-110 ± 3‰ VSMOW), whereas phytoplankton was more ²H depleted (-147 ‰ to -120 ‰) and controlled by river water temperature. These signatures allowed us to quantify POM composition over the study period using a two-component H isotope mixing model. The H isotope results highly compared with the previous C- and N-based mixing models, indicating new potential for H isotopes in studies of POM flux dynamics