The photochemical deactivation process of HCFC-133a (C2H2F3Cl) was investigated by computing excited-state properties with a number of single-reference methods, including coupled cluster to approximated second order (CC2), algebraic diagrammatic construction to second order (ADC(2)), and time-dependent density functional theory (TDDFT). Excited states calculated with these methods, especially TDDFT, show good agreement with our previous multireference configuration interaction (MR-CISD) results. All tested methods were able to correctly predict the properties of the main series of excited states, the n-σ*, n-4p, and n-4s. Nonadiabatic dynamics in the gas phase considering 14 electronic states was simulated with TDDFT starting at the 10 ± 0.25 eV spectral window, to be compared to experimental data measured after 123.6 nm excitation. The excited-state lifetime is 137 fs. Internal conversion to the ground state occurred through several different reaction pathways with different products, including atomic elimination (Cl, F, or H), multifragmentation mechanisms (Cl+F, Cl+H, or F+H), and CC bond-fission mechanisms (alone or with Cl or H elimination). The main photochemical channels observed were Cl, Cl+H, and Cl+F eliminations, representing 54% of all processes.