In the Snake River, an acidic mountain stream in the Colorado Rocky Mountains, the concentration of dissolved iron is apparently unrelated to seasonal changes in hydrologic regime, which strongly influence the concentrations of most other dissolved constituents. Hourly sampling indicated that short-term fluctuations in iron chemistry occur, whereas the concentrations of most other dissolved constituents, including other trace metals, remain stable. During the day, greater concentrations of dissolved total and ferrous iron generally occurred during periods of full sunlight. Photoreduction of hydrous iron oxides, which are abundant in the fine sediment and as coatings on the rocks, may be responsible for these observations. Iron chemistry also varied at night, decreasing in dissolved total and ferrous iron until about midnight and increasing until dawn. Oxidation of ferrous iron, and several microbial processes, may contribute to these nighttime changes. In an on-site batch experiment using rocks and streamwater, an increase in dissolved total and ferrous iron occurred on exposure to sunlight, and ferrous oxidation occurred on return to darkness. Short-term fluctuations in iron chemistry are consistent with the lack of correlation between iron and other constituents in the long-term data, and illustrate the potential importance of complex in-stream processes in such stream systems.