The diffusion of boron from fused borate rods (disodium octaborate tetrachydrate) was monitored over 42 months in CCA-treated Douglas-fir transmission poles. The boric acid equivalent was estimated by the curcumin/salicylic acid color test on increment cores removed from the poles. Moisture content of the poles was quite variable but was always above 20 percent. The percent of increment core length showing boron was also variable with time of sampling from individual poles and between poles. Diffusion of boron increased until 18 months then decreased slightly at 30 and 42 months. Boron was almost always detected downward from the treatment holes at a distance of 25 cm. Likewise, boron was usually detected laterally from the insertion hole at a distance of 7.6 cm. Movement of boron upward from the insertion holes was often nil and not exceeding 5 cm. Thus, because of the variable penetration of boron in the Douglas-fir heartwood, untreated areas are present that are susceptible to decay.

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