A series of polyallylcarbosilane dendrimers and carbosilane-based dendritic polyols up to third generation was analyzed by means of matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass spectrometry and multinuclear NMR spectroscopy to determine the character, origin, and number of structural defects. Besides occasionally reported alpha-hydrosilylation, several side reactions accompanying hydrosilylation, such as isomerization of terminal double bonds, were detected during the synthesis of carbosilane skeleton. Despite increased steric hindrance, internal double bonds react in subsequent addition reactions. Depending on the synthetic sequence applied, the retained reactivity of the internal double bonds can lead either to suppression of the defect in the next generation or to creation of more significant defects such as dendrimer dimers. Hydroboration of allyl groups using dicyclohexylborane proceeded with near quantitative conversion: a small amount of hydrolysis accompanying the following oxidation step producing nonreactive alkyl groups at the periphery was detected.