This paper presents a study of the variance sources and confidence limits in two PLSR models for the determination of the polymorphic purity of Carbamazepine, using near and mid infrared spectroscopy. The variance sources estimated and compared were reference values, instrumental responses and fit of the model. The variance of instrumental responses was estimated experimentally and theoretically, and the differences were discussed. The confidence limits at three confidence levels: 95%, 90% and 50% were determined, presenting a good agreement with the expected values. The predictive ability of the models was compared, showing that both present the same overall performances with RMSEP of 0.67% for near infrared and 0.62% for mid infrared spectroscopy. It was also verified that, for both models, the main variance source remains the error of the PLSR model. (c) 2005 Elsevier B.V. All rights reserved.8015056