The iron(III) azido complex of 5,10,15,20-tetraisopropylporphyrin was characterized with NMR, EPR, Mössbauer, and magnetic susceptibility. These physical methods indicate mixing of the high (S = 5/2) and intermediate (S = 3/2) spin-states of the iron atom. The results were interpreted in terms of the core contraction after nonplanar deformation of porphyrin ring by the bulky isopropyl substituents. In the IR spectrum of this complex, there are two signals at 2062 and 2048 cm-1 due to the antisymmetric vibration of the coordinated azido ligand. The split IR bands demonstrate that the two spin isomers are present, and that the S = 5/2 and 3/2 transition occurs sufficiently slow on the IR timeseale. This is in remarkable contrast with the homogeneous spin-mixing model proposed for the S = 5/2 and 3/2 system. The present observations further suggests that the three S = 5/2, 3/2, and 1 /2 states in iron(III) porphyrin commonly mix through thermal spin equilibrium.