Compost material as biofilter media has been extensively used for contaminant gas treatment in the recent years. However, one of the biggest challenges in the use of this type of material is compost moisture content control. In this work it was observed that for high levels of equilibrium relative humidity (ERH) low levels of equilibrium moisture content (EMC) are obtained (i.e., 99% of ERH produced EMC of approximately 20%, dry-basis, at 25oC). Maintaining high ERH for optimum microbial activity demands a constant water supply through the biofilter irrigation system, considering the low water retention of the material. In order to describe water sorption behavior in compost, seven isotherms equations were evaluated for four selected particle sizes (particles retained in 3.36 mm, 2.38 mm, 2.00 mm, and 1.68 mm openings) of compost biofilters. Observed data were tested against isotherm models for their goodness of fit. Langmuir, Freundlich, Sips (Langmuir-Freundlich), BET, BET for n-layers, GAB, and Henderson isotherms were compared. The Henderson model appeared to provide the best fit, as determined by regression coefficient standard errors and high coefficients of determination for all four particle size ranges tested. In addition, 95% confidence and prediction indicate that the Henderson model can be used for future predictions. The Henderson model was also used to test for significance of particle size treatments. For the four tested particle sizes, the Henderson model parameters for the two largest particle size ranges were not significantly different indicating similar water sorption behavior.