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Factors affecting the reliability of results using 1,10‐phenanthroline (phen) in the quantitative assay of minerals for Fe2+ and total Fe were studied. The greatest source of variability was the photochemical reduction of ferric‐phen species during Fe2+ analyses. It is believed that oxo‐bridged dinuclear complexes of Fe2+ and phen are formed in the mineral digests, which then are susceptible to photochemical reduction by wavelengths < 400–500 nm. The rate of photoreduction of a solution obtained by digesting oxidized nontronite was sufficient to increase the predicted Fe2+ content of the clay from about 0 to 2.45% by simply leaving the sample for 1 hour in normal fluorescent light in the laboratory; and for partially reduced nontronite, from 3.25 to 4.68%. The rates of increase for the two clays were 0.0024 and 0.0014 absorbance units/min, respectively. Interferences due to this phenomenon were eliminated when samples were kept in the dark or under subdued red light. Other factors that adversely influence the accuracy and precision of the standard curve included (i) the amount of 48% HF used to digest the sample; (ii) addition of chemical reducing agents such as hydroxylamine hydrochloride and hydroquinone; (iii) the order in which phen is added relative to other reagents; (iv) pH; and (v) the length of time allowed for color development if phen is added only to the final dilution.