The invention of a portable, lightweigh, rugged, and non-invasive deep tissue pH monitor is proposed. Tissue pH is an acutely sensitive indicator of circulatory demise and tissue perfusion. Currently, arterial bood gas measurments are used to measure tissue pH, but they are relatively slow and require the drawing of a blood sample. A non-invasive deep tissue pH monitor would provide an instantaneous and continuous measure of tissue pH. The pH monitor will use infrared spectroscopy to obtain deep tissue pH information in a non-invasive manner. Multivariate data analysis techniques will be applied to the spectral data to quantify pH information from the spectra. The multivariate technique will e calibrated using standard, well-accepted methods for obtaining pH information and for characterization of this information. The blood and mscle cell samples will be carefully designed such that sample pH values will not correlate with any associated chemical species in the samples that might also have a spectroscopic signature. In Phase II of the research project. Further animal and human studies will be conducted, and the results will facilitate the development of a prototype deep tissue pH instrument.