The gas phase thermal rearrangement of (cyclopropylidene)cyclopropane (bicyclopropylidene) was studied, and the pressure-independent Arrhenius parameters detd. as log(k/s-1) = (14.02+-0.33) - (39.2+-0.7) kcal.mol-1/RT.ln10, almost the same as those for (methylene)cyclopropane. Thus, the second cyclopropane ring in (cyclopropylidene)cyclopropane exerts virtually no influence on the kinetics of the thermal rearrangement of one of its methylenecyclopropane subunits. The rearrangement of (cyclopropylidene)cyclopropane to methylenespiropentane was found to be reversible, with an equil. compn. at 230 DegC of 1:166. These findings are discussed and accounted for. When heated to 350 DegC in a flow system, (cyclopropylidene)cyclopropane undergoes a clean rearrangement to methylenespirocyclopentane [90% yield, >=95% purity on a preparative scale (17.4 g)]. [2-(2-Propenyl)cyclopropylidene]cyclopropane, (2-ethenylcyclopropylidene)cyclopropane, and (E)-(2-methoxycarbonylethenyl)bicyclopropylidene were prepd. from (cyclopropylidene)cyclopropane in 50, 45, and 69% overall yields, resp. The thermal rearrangements of [2-(2-propenyl)cyclopropylidene]cyclopropane and (E)-(2-methoxycarbonylethenyl)bicyclopropylidene proceeded smoothly at 150-160 DegC, resulting in the formation of 4-methylenespiro[2.4]hept-5-ene and its deriv. in 79 and 50% isolated yields, resp., as the sole products, while the rearrangement of [2-(2-propenyl)cyclopropylidene]cyclopropane required higher temps. (200 DegC) and resulted in a complex mixt. of isomeric substituted methylenespiropentanes. Under flash vacuum pyrolysis conditions at 550 DegC, 4-methylenespiro[2.4]hept-5-ene initially formed from (2-ethenylcyclopropylidene)cyclopropane via 1-ethenyl-2-methylenespiro[2.2.]pentane, underwent a further vinylcyclopropane-to-cyclopentene rearrangement to yield a 1:1 mixt. of isomeric tetrahydropentalenes, i.e., 1,2,3,4-tetrahydropentalene and 1,2,3,5-tetrahydropentalene. This constitutes a novel and easy access to the tetrahydropentalene skeleton.