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Abstract— The electron spin resonances of the metastable triplet states of DL‐5‐hydroxy‐tryptophan, 6‐hydroxynicotinic acid, and other related compounds have been observed in various glasses at 77°K. These compounds are biologically interesting in connection with photochemical processes. The Δm= 2 transitions were observed and the spin‐spin interaction parameter (D2+ 3E21/2 was determined. However, for indole, it was also able to observe the Δm= 1 transitions from which D and E were determined separately. From the values of (D2+ 3E2)1/2, D and E whenever possible and from the decay times of the triplet states, it was possible to characterize the lowest triplets as π—π* states in all cases. Studies of the pH dependence of DL‐5‐hydroxytryptophan, 5‐hydroxyindole, and 6‐hydroxynicotinic acid indicate that the unionized molecule exhibits a resonance different from that of the related ion with a proton removed from the phenolic hydroxyl group. The zero‐field splitting parameters D* and the lifetime of the triplet states have also been measured in various glasses and in Lucite. Both temperature and the specific nature of the host media affected the values of these parameters.