It has been known for over 35 years that commonly used sorption isotherm models fail to correctly predict wood–water properties such as heat of sorption. Despite this, their use to determine thermodynamic quantities and monolayer moisture contents persists and in fact is increasing in frequency. In this paper, we recommend the use of the “ABC isotherm,” which is mathematically equivalent to sorption isotherm models commonly used for wood but has the added benefits of simplicity, avoidance of a conceptual image of water sorption that is contradicted by measurements, and avoidance of any additional step of identifying quantities that are physically incorrect.