By means of ns laser flash photolysis, the absorption spectra and molar absorption coefficients (ε) of the 1,4-naphthosemiquinone radical (NQH˙) and 2,3-dimethyl-1,4-naphthosemiquinone radical (DMNQH˙) have been determined to be 8200 dm³ mol^–1 cm^–1 at 365 nm and 7100 dm³ mol^–1 cm^–1 at 368 nm in acetonitrile (MeCN) and MeCN–H₂O (4 : 1 v/v) at 295 K. On the basis of the determined absorption spectra and ε values, hydrogen atom abstraction (HA) by triplet 1,4-naphthoquinone (NQ) and 2,3-dimethyl-1,4-naphthoquinone (DMNQ) from phenol (PhOH) in MeCN was studied by laser photolysis techniques. It was found that HA by triplet NQ and DMNQ (³NQ* and ³DMNQ*) from PhOH occurred in a collision process with quenching rate constants, k_q= 8.6 × 10⁹ and 5.5 × 10⁸ dm³ mol^–1 s^–1, respectively. On the basis of the obtained values of k_q and quantum yields (Φ_HA), the efficiencies (ψ_HA) for HA by both ³NQ* and ³DMNQ* from PhOH were obtained to be unity. The rate constants (k_HA) for HA of ³NQ* and ³DMNA* were determined to be 8.6 × 10⁹ and 5.5 × 10⁸ dm³ mol^–1 s^–1, respectively. The difference in k_HA may be derived from (1) the steric hindrance by the methyl groups of DMNQ and (2) the degree of ³(π,π*) character mixed in with the ³(n,π*) of NA and DMNQ. The triplet–triplet absorption spectra of NQ and DMNQ were identified, and their absolute molar absorption coefficients (ε^T–T_λ) were determined to be 8200 dm³ mol^–1 cm^–1 at 365 nm and 5200 dm³ mol^–1 cm^–1 at 368 nm, respectively, in MeCN on the basis of the quantum yields of intersystem crossing (Φ_isc; 0.74 for NQ, 0.98 for DMNQ) evaluated by thermal lensing techniques. The deactivation mechanism of ³NQ* and ³DMNQ* in the absence of PhOH was found to be self-quenching by NQ and DMNQ in MeCN with the rate constants, k_SQ= 9.7 × 10⁸ and 1.2 × 10⁸ dm³ mol^–1 s^–1, respectively.