Mercury porosimetry was used to study changes in the pore structure of three western Canadian low-rank coals treated in three different atmospheres (nitrogen, steam and combustion products) at temperatures up to 500-degrees-C. The intrusion curves are well described by two different power laws in the pressure range 0.1-200 MPa. Principal-component analysis was performed to determine relations among the power-law exponents, equilibrium moisture and volatile matter.