Evaporation of silicon and consecutive cocondensation of the isolation of three new C 2 H 4 Si isomers [apart from the known silacyclopropene (7) and silylacetylene (8)], namely generated atoms with acetylene or ethylene in an argon matrix turnes out to be a new access to C 2 H 2 Si and C 2 H 4 Si silacyclopropylidene (10), s-trans-vinylsilylene (11), and the unsubstituted 1-silaallene ( 13). These species can be isomers, respectively. In both cases the silicon atoms rather add to the π system than insert into C-H bonds, leading to identified by comparison of their experimental IR spectra and those obtained by ab initio calculations. In a second reaction cyclic silylenes as primary reaction products. In the Si/ acetylene system the primary product is silacyclopro-path, SiH 2 , which is formed as a byproduct from atomic silicon and hydrogen impurities, adds to the π system of penylidene (2), which shows the already known photochemical interconversion into additional C 2 H 2 Si species. On acetylene, yielding silacyclopropene (7). Silacyclopropane ( 16), the analogous product of the reaction of SiH 2 with the other hand, cocondensation of silicon atoms with ethylene and subsequent irradiation lead to the matrix-ethylene, could not be observed.