A mathematical model describing the sulfidation of a single calcined limestone particle was developed and experimentally verified This model, which includes no fitting parameters, assumes a calcined limestone particle to consist of spherical grains of various sizes that react with H2S according to the classic shrinking-core model. The initial size distribution of the grains is derived from mercury porosimetry. The transport of H2S through the bidisperse limestone particle is calculated based on the random-pore model of Wakao and Smith, which distinguishes macropore and micropore zones. Knudsen diffusivity inside the micropore zones is calculated according to the dusty-gas approach. The single-particle model delivers the value of a new defined utilization factor, which includes effects of external mass-transfer limitation, pore-diffusion limitation, and grain-size distribution on particle reactivity. A correlation derived for a single batch of calcined limestone explicitly expresses this utilization factor. as a function of conversion and relevant process parameters. This correlation can be easily incorporated into reactor models, as shown for an existing model describing the capture of H2S by a fluidized bed of calcined limestone particles.