By means of a three-level kinetic model developed previously, the nascent fine-structure branching ratios of K 4 P-2(J) doublets following photodissociation of KI at 248 nm can be determined accurately in the presence of foreign gases. With the forward and reverse Ar-induced collisional fine-structure mixing cross sections of 15 and 30 angstrom2 reported by Lijnse, and our measurement of Ar quenching cross section 0.81 +/- 0.08 angstrom2, the nascent branching ratio of the K 4 P-2(3/2) component is determined to be 0.611 +/- 0.002. Analogously, with the N2-induced collisional mixing cross sections of 100 and 190 angstrom2 reported by Lijnse and Hornman, and our measurement of N2 quenching cross section 18 +/- 2 angstrom2, the branching ratio of K 4 P-2(3/2) is determined to be 0.608 +/- 0.002. The agreement between these values confirms reliability of the kinetic model. However, a lack of confirmity is found in the presence of H-2 quencher, using the collisional mixing cross sections of 53 and 75 angstrom2 reported by McGills and Krause, and our quenching cross section measurement of 2.8 +/- 0.3 angstrom2. The discrepancy is caused by the unsuitable adoption of mixing cross sections. Using the average branching ratio 0.610 determined from the cases of Ar and N2, the H-2-induced collisional mixing cross sections can be evaluated to be 51 +/- 4 and 90 +/- 7 angstrom2. The obtained quenching cross sections by collisions with foreign gases are consistent with those reported elsewhere except for the case of Ar. The discrepancy of our Ar measurement of 0. 81 +/- 0.08 angstrom2 from a reported value < 0.07 angstrom2 does not cause a significant difference in the branching ratio determination; it is because the magnitude of the mixing cross sections are more than ten times larger than the relevant quenching cross sections, such that the dependence on the collisional quenching becomes insignificant.