Nuclear magnetic resonance spectroscopy is used frequently for structural characterization of humic substances (HS). However, the study of relaxation phenomena of HS, including spin-lattice and spin-spin relaxation, has received little attention. The present study investigated proton spin-lattice relaxation times (T1) of humic acids (HA) in deuterated 0.5 M sodium hydroxide (NaOD) and dimethylsulfoxide (d6-DMSO) at various concentrations, temperatures, and pHs. The two HAs used were extracted from an Amherst peat soil and a New Hampshire peat soil. Overall, the proton T1s of humic acids used in this study ranged from 0.3 to 0.8 s in d6-DMSO, and from 0.26 to 2.3 s in 0.5 M NaOD/D2O. Aromatic protons had much longer T1than aliphatic protons in 0.5 M NaOD; however, both proton types exhibited similar T1values in d6-DMSO. These results indicate that humic acids might have different conformations in the two solvents. Proton T1generally decreased with increasing HA concentration. Only protons from methyl and methylene/methine groups exhibited a slight T1increase, with a temperature increase from 20 °C to 60 °C, with protons from other functional groups displaying a reduced T1, except for aromatic protons of New Hampshire HA in 0.5 M NaOD. Moreover, both HAs demonstrated distinct T1changes in alkaline solution as pH decreased. The proton T1values and their changes with environmental conditions can further the interpretation of segmental mobility and molecular conformation of dissolved HAs in different solvents.