Borate penetration relies on diffusion when borate and glycol-borate preservatives are applied to the surface of wood. This study evaluated the extent of borate penetration in framing lumber as a function of preservative formulation, wood moisture content, and diffusion time after treatment. In Phase I of the study, end-matched specimens were conditioned to target average moisture contents of 15, 25, or 35 percent, briefly immersed in borate formulations, and then placed into wooden frames to minimize air exchange during diffusion. Penetration in these specimens was generally less than 5 mm (or 35% of the cross section) regardless of treatment solution, target moisture content at time of treatment, or diffusion period (2, 4, or 8 wk). Assay of boron concentrations after 8 weeks of diffusion also indicated that the boron was concentrated in the outer 5 mm of the wood. Diffusion appeared to have been limited by the relatively rapid drying of the specimens, even with the restricted air movement within the wooden frames. In Phase II of the study, specimens were conditioned to a target average moisture content of 20 percent prior to dip immersion and then placed in a room that maintained an equilibrium moisture content of 19 to 21 percent. Penetration in these specimens was assessed after 6, 13, and 26 weeks of diffusion. After 6 weeks of diffusion, average boron penetration exceeded 5 mm, and after 26 weeks of diffusion, penetration exceeded 11 mm, or over 70 percent of the cross section. Little difference in diffusion was observed between the types of borate formulations evaluated in either phase of this study. The results of this study indicate that rapid drying conditions may limit penetration of boron from spray applications; however, in situations where high humidity is maintained in a structure, substantial diffusion is possible.