The emission spectra of the A2Π–A′2Δ transition of LaS have been reinvestigated using spectra recorded with the 1-m Fourier transform spectrometer of the National Solar Observatory at Kitt Peak. Sixteen bands involving the vibrational levels v′=0–5 and v″=0–5 of the Δv=−1, 0 and 1 sequences of the A2Π3/2–A′2Δ5/2 sub-band have been rotationally analyzed to derive an improved set of spectroscopic constants for the two states. Rotational perturbations have been observed near J=171.5, 155.5, 134.5, 112.5, 85.5, and 43.5, respectively, in the v=0, 1, 2, 3, 4, and 5 vibrational levels of the A′2Δ5/2 state. These perturbations are caused by interaction between the A′2Δ5/2 (v) and A′2Δ3/2 (v+2) levels of the lower state.