Measured moisture conditions at several existing field research sites could not be related to precipitation, and were thought to be temperature-dependent. Accordingly, subsurface thermistor probes were installed at six field research sites, to measure subgrade temperature gradients and relate them to measured subgrade moisture conditions. After a freview of previous work in thermal moisture flow and subgrade temperature measurement by other agencies, instrumentation and procedures for measurement of subgrade temperatures under existing highway pavements are described, and procedures for data collection and evaluation are given. Results of the research showed that temperature gradients caused measurable subgrade moisture migration, but, in Oklahoma subgrades, the magnitude of temperature-induced moisture variation appears to be of secondary nature, when compared to other factors cuasing subgrade moisture variations. It is possible, however, that temperature-induced moisture migration and resulting heave may be sufficient to cause initial pavement cracking. Once the pavement is cracked, large precipitation-dependent subgrade moisture variations occur, producing rapid pavement deterioration.