This paper is the first illustration of an innovative procedure for studying the dynamics of condensed phases in hydrothermal carbonization of lignocellulosic waste. A novel technique, based on a particular implementation of van Krevelen plots, is shown to be capable of monitoring the compositions precisely during this process. Two fundamental laws describe the system evolution: double-exponential decay and a modified form of the Hill equation. The modeling was tested using batch experiments (200 degrees C, up to 2 h) performed with three different lignocellulosic materials (silver fir, beech wood, and olive pruning). A high goodness of fit (R-2 as high as 0.999) supports the approach and heuristically indicates the way for further research. The method allows for prediction of the time course of solid yield, heating value, atomic ratios, and other relevant process variables. The technique also provides valuable and easy-to-obtain information to address the process optimization of industrial plants.