This is digital research data corresponding to a published manuscript in "Comparative studies on water- and vapor-based hydrothermal carbonization: process analysis" Energies 2020, 13, 5733. Dataset may be assessed via the included link at the Dryad data repository. Hydrothermal carbonization (HTC) reactor systems used to convert wet organic wastes into value-added hydrochar are generally classified in the literature as liquid water-based (HTC) or vapor-based (VTC). However, the distinction between the two is often ambiguous. In this paper, we theoretically developed models for predicting reactor pressure, volume fraction of liquid water and water distribution between phases as a function of temperature. Methods are described in the manuscript http://dx.doi.org/10.3390/en13215733. Descriptions corresponding to each figure and table in the manuscript are placed on separate tabs to clarify abbreviations and summarize the data headings and units.