In the present study, the second order structures of the 16PF-5 for 3, 4, 5 and 6 factors are analysed in a sample of 636 undergraduate students and their friends and relatives, with a mean age of 25.09 years (sd: 9.20). A two-stage analysis with Exploratory and Confirmatory Factor Analysis (EFA and CFA) was performed assigning subjects either to a calibration sample (314 subjects), or to a validation sample (322 subjects). First, EFAs were conducted for each of the 4-factor structures with the calibration sample, and second, CFAs were specified and assessed also for the 4-factor different structures. Four different situations were tested, simple structure, secondary loadings greater than 0.30, modification indices between covariance error terms greater than 20, and modification indices between covariance error terms greater than 15. Results show that the 16PF-5 simple structure yields poor fit indices throughout all models, although when incorporating secondary loadings and correlating the variances of the error terms with modification indices greater than 15, the 5-factor model appears to be the best representation of observed data. The usefulness of CFA in the study of personality structural questionnaires is discussed in the light of these results. (Contains 2 tables.)