The accurate measurement of tissue mineral density, ρ_m, in specimens of unequal size or quantities of bone mineral using polychromatic μCT systems is important, since studies often compare samples with a range of sizes and bone densities. We assessed the influence of object size on μCT measurements of ρ_m using (1) hydroxyapatite rods (HA), (2) precision-manufactured aluminum foams (AL) simulating trabecular bone structure, and (3) bovine cortical bone cubes (BCt). Two beam-hardening correction (BHC) algorithms, determined using a 200 and 1200 mg/cm³ HA wedge phantom, were used to calculate ρ_m of the HA and BCt. The 200 mg/cm³ and an aluminum BHC algorithm were used to calculate the linear attenuation coefficients of the AL foams. Equivalent ρm measurements of 500, 1000, and 1500 mg HA/cm³ rods decreased (r² < 0.96, p < 0.05 for all) as HA rod diameter increased in the 200 mg/cm³ BHC data. Errors averaged 8.2% across these samples and reached as high as 29.5%. Regression analyses suggested no size effects in the 1200 mg/cm³ BHC data but differences between successive sizes still reached as high as 13%. The linear attenuation coefficients of the AL foams increased up to approximately 6% with increasing volume fractions (r² < 0.81, p < 0.05 for all) but the strength of the size-related error was also BHC dependent. Equivalent ρ_m values were inversely correlated with BCt cube size (r² < 0.92, p < 0.05). Use of the 1200 mg/cm³ BHC ameliorated the size-related artifact compared to the 200 mg/cm³ BHC but errors with this BHC were still significant and ranged between 5% and 12%. These results demonstrate that object size, structure, and BHC algorithm can influence μCT measurements of ρ_m. Measurements of ρm of specimens of unequal size or quantities of bone mineral must be interpreted with caution unless appropriate steps are taken to minimize these potential artifacts.