Methods and apparatus are provided for non-invasively determining blood pH by a multivariate analysis technique, such as classical least squares (CLS), partial least squares (PLS), or principal components regression (PCR). The technique employs at least one spectral factor based on the difference in the spectrum of a hemoglobin fraction (e.g., oxyhemoglobin) at two different pHs (e.g., pH 7.2 and pH 7.8). The spectral factor is preferably evaluated oover the wavelength range of 520-680 nm.