This study examines the influence of analytical particle size on the surface area and mesopore and micropore volume data obtained from low-pressure N-2 and CO2 adsorption analyses in response to the crushing of coal and shale. Pennsylvanian high-volatile bituminous coal (R-o similar to 0.57%) and Devonian to Lower Mississippian low maturity (R-o similar to 0.57%) and high-maturity (R-o similar to 1.30%) shales from the Illinois Basin were progressively crushed from chunks (similar to 7 mm) to 4 mesh (< 4.78 mm), 7 mesh (< 2.83 mm), 18 mesh (< 1 mm), 30 mesh (0.595 mm), 60 mesh (< 0.250 mm), 200 mesh (< 0.074 mm), and 230 mesh (< 0.063 mm), and, subsequently, low-pressure N-2 and CO2 adsorption analyses were performed on all the grain size fractions. Our results demonstrate that the values of both surface area and specific mesopore and micropore change with progressive crushing. For example, BET surface area of coal shows a steady increase from 2 m(2)/g in the 4 mesh fraction to 4.7 m(2)/g in the 200 mesh fraction. For comparable size ranges, BET surface area changes from 0.15 to 7.82 m(2)/g in the low maturity shale, and from 0.02 to 6.26 m(2)/g in the high-maturity shale. Changes in mesoporosity and microporosity parameters indicate that the coarsest fractions (4 mesh and larger) are not suitable for low-pressure adsorption analysis; the values are very low and not reproducible dominantly because of equilibration problems. Our results demonstrate that the 60 mesh fraction for coal and the 200 mesh fraction for shales seem to be optimal and the most practical sizes for performing low-pressure N-2 and CO2 adsorption analysis; these analytical particle sizes yield results closest to the "real" values, unbiased by disequilibrium.