Complexes trans-[PtRCl(SR'(2))(2)], where R = Ph, mesityl, and p-anisyl and R' = Nle or Et, have been synthesized and their crystal and molecular structures determined. Crystals of trans-[PtPhCl(SEt2)(2)] (2) are triclinic () with a = 10.112(4) Angstrom, b = 13.158(2) Angstrom, c = 14.714(5) Angstrom, alpha = 102.48(2)degrees, beta = 94.394(4)degrees, gamma = 90.22(3)degrees, and Z = 4. Crystals of trans-[Bt(mesityl)Cl(SMe2)(2)] (4) are monoclinic (P2(1)/c) with a = 13.158(2) Angstrom, b = 9.170(1) Angstrom, c = 16.013(3) Angstrom, beta = 120.93(2)degrees, and Z = 4, and crystals of [Pt(p-anisyl)Cl(SMe2)(2)] (5) are monoclinic (P2(1)/n) with a = 9.879(4) Angstrom, b = 8.128(2) Angstrom, c = 19.460(5) Angstrom, beta = 96.56(3)degrees, and Z = 4. All complexes are square-planar, featuring Pt-CI distances between 2.40 and 2.42 Angstrom, indicating a large ground-state trans influence of the aryl group. The coordination geometry is maintained in methanol and chloroform solution as shown by H-1-NMR spectra. The kinetics of substitution of the labile chloride trans to aryl by various nucleophiles has been studied in methanol by variable-temperature and -pressure stopped-flow spectrophotometry. A two-term rate law with a well-developed solvolytic pathway is followed. Negative entropies and volumes of activation indicate an associative mode of activation in all cases, independent of steric blocking of the axial sites and a large Pt-CI ground-state bond-weakening. Comparison of the reaction rates of the present series of complexes with their bis(phosphine) analogues and with related cyclometalated compounds shows that the triethylphosphine complexes are 2-3 orders of magnitude less reactive than the thioether complexes, which in turn are a factor 10-20 less reactive than the cyclometalated ones. This reactivity increase can be rationalized mainly in terms of a decrease in steric hindrance in the series. There seems to be no inherent differences with regard to trans labilizing ability of the aryl ligands in the various types of complexes, including the cyclometalated ones.