Intelligence differences might contribute to true differences in personality traits. It is also possible that intelligence might contribute to differences in understanding and interpreting personality items. Previous studies have not distinguished clearly between these possibilities. Before it can be accepted that scale score differences actually reflect personality differences, personality items should show measurement invariance. The authors used item response theory to test measurement invariance in the five-factor model scales of the International Personality Item Pool (IPIP) and NEO-Five-Factor Inventory (NEO-FFI) across two groups of participants from the Lothian Birth Cohort 1936 with relatively low and high cognitive abilities. Each group consisted of 320 individuals, with equal numbers of men and women. The mean IQ difference of the groups was 21 points. It was found that the IPIP and NEO-FFI items were measurement invariant across all five scales, making it possible to conclude that any differences in IPIP and NEO-FFI scores between people with low and high cognitive abilities reflected personality trait differences. (Contains 5 tables.)