It is not uncommon to observe that vitrinite retlectance results from coal samples with equal maturity and type of vitrinite are distributed over a range of up to 0.20-0.25 %Rm for maturity levels around 0.5 to 1.0 %Rm. It is demonstrated that the variations may be related to microlithotype composition of the samples and accordingly to the depositional environment. A practical consequence is that uncritical interpretation of data may gi ve misleading results. However, improved precision can be obtained by increasing the number of samples, and by considering microlithotype composition.