The time-resolved enthalpy and the structural volume changes after excitation of native oat phytochrome A were studied in the micro- to milliseconds range by photothermal beam deflection (PBD), a technique that follows the time-resolved refractive index changes upon decay of the excited species. The first set of intermediates, I₇₀₀¹ and I₇₀₀², stores ca 83% of the energy of the first excited state, in agreement with previous optoacoustic data, whereas the second set stores only ca 18%. The temperature dependence of the amplitudes ratio for the optical absorbances of the (I₇₀₀¹ I₇₀₀²) intermediates set is explained on the basis of the thermochromic equilibrium between P_r,657 and P_r,672, which also is in line with the present PBD data. These data were best fitted with a parallel mechanism (with equal yield in each branch) for the production of the first set of intermediates, I₇₀₀¹ and I₇₀₀², as well as the second set of intermediates, I_bl¹ and I_bl². Thus, the final steps toward P_fr should be largely driven by positive entropic changes brought about by protein movements, in line with previous resonance Raman data. For the production of the first set of intermediates (I₇₀₀¹ and I₇₀₀²) an expansion of 18 ± 13 mL mol⁻¹ was determined, and a further expansion ≥7 mL mol⁻¹ was estimated for the decay from I₇₀₀ⁱ to the set of I_bl intermediates, indicating that the far red–absorbing form of phytochrome (P_fr) has a larger volume than the red-absorbing form of phytochrome. This is in agreement with previous chromatographic and circular dichroism data according to which P_fr shows a larger volume and the chromophore shows a higher accessibility, respectively, in the P_fr state.