This note investigates the computation of extents of reaction from a limited number of state measurements. It is shown that this computation differs depending on whether the transformation to vessel extents is based on the heat or the enthalpy balance. Accordingly, two different ways of implementing these computations are described and compared, where the first one corresponds to a linear time-invariant transformation, while the second one involves a novel linear time-varying transformation. Furthermore, it is shown that, if the component molar enthalpies depend linearly on temperature, the two transformations yield similar results. (C) 2020 Elsevier Ltd. All rights reserved.