The K-rotational relaxation of electronically excited S 1 H 2 CO from the V 4 ) 1, J K a K c ) 1 01 rovibronic state with S 0 H 2 CO as the collider has been investigated. Dispersed fluorescence spectra and population time profiles of individual K a ′ parent and satellite states have been recorded. Curvature in the Stern-Volmer plot is qualitatively discussed. Quantitative analysis of total and state-to-state K-rotational relaxation processes is carried out by two independent analytical kinetic schemes, a decoupled model and a coupled model. The results reveal the dominance of the ∆K a ) (2 step and a minor contribution from the ∆K a ) (4 step. The rate of pure K-collisional relaxation is determined to be (3.1 ( 0.8) × 10 7 s -1 ‚Torr -1 , accounting for 28% of the total collisional relaxation rate of S 1 H 2 CO. Mechanisms of collisional depopulation from a single excited rovibronic state are reviewed to elucidate the overall collisional relaxation dynamics of S 1 H 2 CO.