Abstract At pH 11, 22 ± PC, larger percentage-yields (%-yields) of hurnic acids (HAs) were obtained from catechol than from hydroquinone after 1 day, and the reverse after 4 days. Maximum %-yields were obtained at 3 days for catechol and 4 days for hydroquinone; both remained asymptotic thereafter at %-yields of 44% and 86% respectively. Percentage-yield was independent of initial concentration of reactants over the ranges 1.6–6.4 m catechol and 0.3–1.2 M hydroquinone, ranges that took into consideration solubility limitations. Significant increases in %-yields were obtained at each pH step from 7 to 11. and significant amounts of HAs derived from hydroquinone (H-HAs) but not HAs derived from catechol (C-HAs) were produced at pH 5 and pH 7. Energies of activation were .8786 and 12,134 Jdeg − mol −for catechol and hydroquinone respectively. Similar %-yields were obtained at 75° and 50°C, which were significantly higher than those obtained at 25°C. Without continuous shaking, %-yields of C-HAs were 2.0, 3.2 and 4.1 in darkness. Vis. and u.v. radiation respectively; constant %-yield, 26.8 ± 3.7 (SE), was obtained for H-HAs in darkness or under Vis. or u.v. Shaking did not increase %-yields of H-HAs, but significantly increased those of C-HAs—3.5-,3. 6- and 5.5-fold in darkness. Vis. and u.v. Hydrogen peroxide (0.5–3 M) had a stimulatory effect on synthesis of C-HAs during 24 h, with significantly increased production of C-HAs when present at 3 M. Production of H-HAs was inhibited throughout this range of H 2O. C-HAs were less soluble than H-HAs at pH 7, had a higher molecular weight, and possessed a lower concentration of free radicals when formed after 1 day but a higher concentration when formed over 4 days. Significant amounts of biomass from Saccharomyces cererisiae (yeast) were assimilated in 4 days by H-HAs at pH 7.0. Evidence was not obtained to determine whether this could be attributed to a destructive effect of the H-HAs with subsequent uptake of both soluble degradation products and autolyzed soluble components of the yeast cells, or no destruction but mere uptake of the latter only. Reasons are provided to suggest that the induction of the initiation step of polymerization, or the maintenance of the propagation step, are fundamentally different for catechol and hydroquinone.