Fluorescence intensity in thioformaldehyde vapors (H 2 CS), excited to the A 1 A 2 different vibronic levels of the A 1 A 2 <-X 1 A 1 transition, were measured as a function of an external magnetic field. On excitation to these levels, dynamics in zero and nonzero field may be described in the small-molecule limit, with fluorescence exhibiting an almost exponential decay. A magnetic field changes the integrated intensity and decay lifetime of the thioformaldehyde fluorescence induced from different vibronic levels of the A 1 A 2 state. We found that the magnetic field effect grows at lower gas pressures. The measured field dependences of the magnetic field effect can be fitted using field-saturated functions. The measured data were explained by the indirect mechanism theory (nuclear-spin and electron-spin decoupling mechanism).