We report a novel synthesis of the cyclic oligomers [(H2C=CH) (CH3)SiO](n) obtained by the reaction between phosphorous pentachloride and methylvinyl diethoxysilane. According to gas chromatography/mass spectrometry data, the reaction product consisted of a mixture of cyclic oligomers in which the most important fractions were composed of cycles with n = 5, 6, or 7. The reaction product was also characterized by Fourier transform infrared and H-1 and C-13 NMR spectroscopy. Experimental results suggested a new kind of reaction between the phosphorous pentachloride and terminal olefins directly bonded to silicon, which was probably associated with sterical effects favoring C-O-Si bond cleavage of ethoxy groups instead of the conventional addition of phosphorous pentachloride to olefinic linkages.