The extinction coefficient ratio (ECR) of coordinatively bonded pyridine (Lpy) and protonated pyridine (Bpy) was determined in situ by diffuse-reflectance FTIR spectroscopy (DRIFTS), by exposing sulfated zirconia (SZ) containing chemisorbed pyridine to water vapor. Both pure and KBr-diluted SZ samples were analyzed. The linearity with concentration of the Kubelka-Munk transformation was verified for pyridine interacting with the undiluted SZ samples. The portability of published ECR values between transmission and diffuse-reflectance IR spectroscopy, as well as between different types of materials, was found questionable. For the SZ samples analyzed, following a statistical analysis of the data, an Lpy/Bpy ECR value of 2.05 +/- 6.3% was determined, resulting in the calculation of an initial Lewis/Bronsted acidity ratio value of 1.1 in the freshly activated material. This ECR determination method should be applicable to other similar solid acids that have a window of relative transparency in the 1600-1400 cm(-1) range and are capable of retaining chemisorbed pyridine upon exposure to water vapor.