In considering variations in the radiocarbon content of natural material, it is important to distinguish between processes of fractionation and of dilution; thus the accumulated data on C13 variations can be useful if correctly interpreted. In this paper the mechanism of the isotopic fractionation in plants is discussed, in an attempt to evaluate the relative effects of the local environment and of the activities of the plants themselves. It is shown that "cyclic enrichment processes" have had no effect on the isotopic composition of the various groups of plants. The relationships between C13 and C14 variations in depositional processes of carbonaceous materials are discussed with respect to the relative enrichment of these isotopes in carbonates, and an estimated "age" is derived for the carbon in the ocean. The established ranges of C13 concentration in the various types of carbonaceous materials allow accurate estimates to be made of the errors which can be introduced in radiocarbon dating by natural i...