Irradiation of phytochrome solutions with a high-intensity mixed red and far red light source causes measurable absorbancy increases at 543 nm. Evidence is presented that these absorbancy increases are caused by accumulation of intermediates on the P(R) to P(FR) pathway with relatively slow thermal decay constants. Kinetic analysis of the decay signals is consistent with the interpretation that the signals represent simultaneous independent and parallel decay of 2 species by first order kinetics to P(FR). If actinic light intensity is kept constant and exposure time changed, the relative amounts of the 2 components change, with proportionately more of the rapidly decaying species present following short exposure times. If the amount of the intermediates is decreased by decreasing actinic light intensity at constant exposure time, however, the relative amounts of the 2 remain constant. The Q(10) for intermediate decay following illumination is approximately 2.0, while that for complete phototransformation of the pigment in either direction is very close to 1.0. Incomplete transformation of P(R) to P(FR), caused by overlapping absorption of the 2 forms, is shown by the presence of intermediates (indicating cycling of the pigment) in continuous red light. Such intermediates do not appear in continuous far red, indicating a rate of pigment cycling below detection by the available instrumentation.