Thermal decomposition of bis(trialkylphosphine)-3,3,4,4-tetramethylplatinacyclopentanes in hydrocarbon solvents yields two major products: 2,2,5p-tntramethylbutane and l-methyl-l-tert:butylcyclopropane. The former appears to be generated by heterogeneous process(es) catalyzed by platinum(0) (colloidal and/or solid) formed during the decomposition; the latter is the product of a homogeneous reaction sequence. The existence of competing heterogeneous and homogeneous reaction pathways complicates the study of this thermal decomposition. The addition of mercury(O) to the system selectively suppresses the heterogeneous reaction by poisoning the bulk platinum(O). This technique has been tested in one well-defined model system-the homogeneous hydrogenation of dineopentylbis(triethylphosphine)platinum(Il) to dihydridobis(triethylphosphine)platinum(Il) and neopentane in the presence of the heterogeneous platinum(0)-catalyzed hydrogenation of l-methylcyclopentene to methylcyclopentane-and found to eliminate the heterogeneous reaction while leaving the homogeneous one essentially unaffected. Application of this technique to the thermal decomposition of 3,3,4,4-tetramethylplatinacyclopentane simplifies the reaction by eliminating 2,2,3,3-tetramethylbutane as a product. Similarly, addition of mercury(0) to solutions containing H2 and (1,5-cyclooctadiene)dimethylplatinum(Il) in cyclohexane suppresses the autocatalytic, heterogeneous platinum-catalyzed conversion of the organoplatinum compound to (inter alia) cyclooctane, methane, and platinum(0). In other reactions examined-especially the high-temperature thermal decompositions of (1,5-cyclooctadiene)dineopentylplatinum(II) and of cis-bis(cyclopentyldimethylphosphine)dimethylplatinum(Il) in hydrocarbon solvents-mercuq/ poisoning does not successfully separate homogeneous and heterogeneous reactions. In the latter instance, an additional complicating processapparent reaction of mercury(O) with the organoplatinum compound-introduces reaction paths which seem to generate methyl radicals. The paper includes a simple preparation of 1,4-dihalo-2,2,3,3-tetramethylbutane and illustrates the use of this material as a reagent for the preparation of 3,3,4,4-tetramethylmetallacyclopentanes via the corresponding di-Grignard reagents.