This study describes the development of near infrared (NIR) calibration models using transmittance measurements in powder samples and compares the results obtained with those of tablet transmittance and diffuse reflectance of powders. Transmission near infrared spectroscopy is a method widely used for the analysis of tablets in the evaluation of drug concentration due to the larger sample volume analyzed, but not commonly used for the analysis of powder samples. Diffuse reflection near infrared spectroscopy is a method used in both powder and tablets for the evaluation of quality attributes. In this initial study NIR transmittance measurements were obtained using an off-line spectrometer equipped with a high intensity light source. Spectra were obtained with three different resolutions for the analysis of powder and tablet samples of 7.50–22.50% (w/w) acetaminophen. The Partial Least Squares (PLS) calibration models developed include pretreatments such as Standard Normal Variate (SNV) and first derivative in the region from 9500–7500 cm−1. Transmittance in powder presented low Root Mean Square Error of Prediction (RMSEP) values that varied from 0.23–1.15% (w/w) APAP with resolution of 64 and 16 cm−1. The lowest RMSEP values (0.23–0.39% (w/w) APAP) were obtained using a resolution of 64 cm−1. The RMSEP values for powder transmittance measurements were 2.4–5.6 times lower than the diffuse reflectance measurements of the powder mixtures.