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 mercuiy porosimetiy. The transport of H2S through the bidisperse limestone particle is calculated based on the randompore 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 correlatiori 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.