The K-rotational relaxation of electronically excited S-1 H2CO from the v(4) = 1, J(KaKc) = 1(01) rovibronic state with S-0 H2CO 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 Delta K-a = +/-2 step and a minor contribution from the Delta K-a = +/-4 step. The rate of pure K-collisional relaxation is determined to be (3.1 +/- 0.8) x 10(7) s(-1) . Torr(-1), accounting for 28% of the total collisional relaxation rate of S-1 H2CO. Mechanisms of collisional depopulation from a single excited rovibronic state are reviewed to elucidate the overall collisional relaxation dynamics of S-1 H2CO.