Many samples of Intsia spp. contain a few vessels which, throughout their length, possess some elements filled with bright yellow crystals. They serve as a characteristic of the genus. An examination was undertaken to elucidate the mode of formation of these crystals, which have been identified as pure robinetin, and the factors which may control their presence. It was concluded they are formed at pit membranes inside these particular vessels. The non-structural components of fibres, parenchyma and vessels in heartwood differ from each other. Also with the possible exception of the fibres, aberrant cells of each tissue formed different compounds. Heartwood extractives are not transferred directly from the parenchyma to other tissues in all species.