Raman spectra of sugi (Cryptomeria japonica D. Don) sapwood treated with aqueous boric acid solutions (5g/dm³-45g/dm³) were recorded using a near infrared laser as an excitation source. The Raman measurements were carried out only on earlywood of cross section of the specimens treated with the solutions. As far as boron species, no Raman band was observed except for B(OH)₃. Weight gain of the sugi specimen treated was roughly proportional to the boric acid concentration. The ν₁ band intensity of B (OH)₃ also increased in relation to the concentration above 15g/dm³, while the band was scarcely detectable owing to low intensity below 10g/dm³. This indicates that there is a discontinuity in variation of the band intensity with the concentration. Herein we attempt to account for the discontinuity in terms of boric acid behavior in wood during the treatment.

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