The existing TMVOC numerical reservoir simulator, developed to model the migration of organic mixtures in the subsurface under multiphase conditions, was improved by adding capabilities for the modeling of aerobic and anaerobic biodegradation reactions of hydrocarbons and chlorinated solvents. Reactive transport is coupled with the multiphase nonisothermal flow of multicomponent fluid mixtures containing water and sets of user-defined noncondensible gases (NCG), volatile organic compounds (VOCs), and dissolved solids. The mathematical formulation of biodegradation reactions, a modified version of that developed for the BIOMOC computer code, is presented together with underlying assumptions. TMVOCBio allows the modeling of simultaneously occurring aerobic and anaerobic degradation processes involving multiple organic substrates, electron acceptors (EA), and nutrients, accounting for the inhibition phenomena conventionally considered by other analytical and numerical codes. Code verification against accurate numerical solutions and code validation against published laboratory and field experimental results relevant to saturated subsurface systems showed good agreement.