Goldberg’s (2001) IPIP Big-Five personality factor markers currently lack validating evidence. The structure of the 50-item IPIP was examined in three different adult samples (total N=906), in each case justifying a 5-factor solution, with only minor discrepancies. Age differences were comparable to previous findings using other inventories. One sample (N=207) also completed two further personality measures (the NEO-FFI and the EPQ-R Short Form). Conscientiousness, Extraversion and Emotional Stability/Neuroticism scales of the IPIP were highly correlated with those of the NEO-FFI (r=0.69 to −0.83, p<0.01). Agreeableness and Intellect/Openness scales correlated less strongly (r=0.49 and 0.59 respectively, p<0.01). Correlations between IPIP and EPQ-R Extraversion and Emotional Stability/Neuroticism were high, at 0.85 and −0.84 respectively. The IPIP scales have good internal consistency and relate strongly to major dimensions of personality assessed by two leading questionnaires.