Soil particle density (ρ _s ) is often assumed as 2.65 g cm ⁻³ (density of quartz). The objectives of this study were to compare the use of different gases for determining ρ _s in a gas pycnometer and relate measured ρ _s to soil particle size distributions. The ρ _s of 36 natural soil samples representing 12 USDA textural classes, fine glass beads, crushed granite rock, kaolinite, and bentonite were measured by a commercial gas pycnometer using He, N ₂ , and dry air. The ρ _s of 30 of the soil samples, glass beads, and crushed rock were also determined with a water pycnometer. The ρ _s of 36 soil samples determined by He and 30 samples determined by water had narrow ranges with averages of 2.65 and 2.59 g cm ⁻³ , respectively. The ρ _s determined by air and N ₂ had much wider ranges with averages of 2.93 and 2.97 g cm ⁻³ , respectively. There was a near 1:1 relationship between ρ _s of all samples determined by air and N ₂ with a highly significant ( p < .001) correlation coefficient ( r = .99). The average ratio of ρ _s determined by He and water was 1.03, but the correlation coefficient for their relationship was only .416. Although the relationship between ρ _s determined by He and either air or N ₂ was relatively strong ( r < .61), the regression coefficient was <.17. There was a strong relationship between soil clay content and ρ _s determined by N ₂ or air but a weaker, yet statistically significant ( p < .05) relationship when using He.