A theory is presented to calculate the penetration depth of light in single crystal or polycrystalline material doped with the molecule whose electronic state can be excited by photoabsorption from the ground state to the excited singlet state, and transferred by intersystem crossing into the triplet state. An experimental study on the penetration depth is also reported, which is made by measuring the amplitude of the zero-field electron spin resonance (ESR) signal of the photoexcited triplet electron spins in single crystal and polycrystalline pentacene-doped p-terphenyl and naphthalene samples at room temperature. The samples were irradiated by a pulsed laser beam, and the zero-field ESR signals were observed for various sample thicknesses. In single crystals of 0.053 mol % pentacene-doped p-terphenyl, the pentacene molecule within ca. 1 mm from the surface of the sample was found to undergo intersystem crossing to the triplet state by the laser irradiation with a pulse duration of 1 mus and a beam intensity of 9.2x10(8) W m(-2). This result could be well reproduced by the calculation using the reported kinetic parameters, and the limit depth of photoexcitation to the triplet state is shown to coincide with the penetration depth of light when the effect of the stimulated emission is negligible. For polycrystalline 0.099 mol % pentacene-doped p-terphenyl, the penetration depth under an incident beam intensity of 2.9x10(9) W m(-2) was determined to be ca. 0.7 mm, which could be reproduced by taking account of the laser beam attenuation due to multiple scattering at the crystallite boundaries in the calculations. For pentacene-doped naphthalene, the experimental results were reproduced by the simulations using the intersystem crossing yield of 40%, which is much higher than the value (2%) reported at 1.4 K.