Background and objectives: The Experiences in Close Relationships scale (ECR) is much used in adult attachment research. In this study we examined: the psychometric properties of the Dutch ECR in comparison with the original American ECR; the claim of orthogonality of its scales Avoidance and Anxiety; and a broader applicability of the ECR, because much research is restricted to psychology student samples. Methods: In Study 1 we investigated the ECR's internal structure by means of confirmative factor analysis in the American psychology student sample in which the ECR was originally validated. We compared these findings with the results of factor analyses in: a Dutch psychology student sample to test whether the Dutch translation yields comparable results; and a Dutch general population sample to evaluate the Supposed orthogonality and to determine a broader applicability of the ECR. In Study 2 we evaluated aspects of the external validity of the ECR. Results: Confirmative factor analysis supports the original two factor structure in both Dutch samples, although less clear in the population sample. As opposed to both student samples the scales correlate moderately in the population sample. Results support external validity of the ECR in both Dutch samples. Conclusions: Since the supposed orthogonality of the scales varies by sample, the internal validity of the (Dutch) ECR varies by sample as well, namely from satisfactorily (population) to adequate (students). The external validity of the Dutch ECR is compared to the American version adequate, suggesting a broader applicability.