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An approach suitable for quantitative description of oxidation of porous powders with complex particle morphology is developed and applied for a 95% pure boron powder. It is shown that boron particles, which are agglomerates of nano-sized, partially fused primary particles, can be represented as spheres with reactive porous shells and impenetrable cores. The oxidation of such powders is assumed to occur volumetrically within the reactive shells. The shell thickness is obtained from simple experiments with the actual powders. To determine the thickness, thermo-gravimetric measurements are performed and processed for powders with different but overlapping particle size distributions. The processing uses measured powder particle size distributions to assign fractions of the powder mass gain to individual particle size bins. Selection of the appropriate initial reactive shell thickness makes it possible to ensure that particles of the same size present in samples with different size distributions oxidize in the same way. The specific powder studied here was successfully described with a reactive shell thickness of about 1.28 µm. Surface area measurements can be used to interpret the reactive shell as comprising packed spherical primary particles. For the powder used here the radius of such primary particles is 80 nm. The proposed reactive shell model is shown to be valid as long as the thickness of boron oxide grown on the primary particles does not exceed 15 nm. The approach is useful for describing reactions leading to ignition of porous powders, and of powders consisting of irregular aggregates in particular. It is also expected to be applicable to describe corrosion, catalyst aging, and other phenomena involving surface reactions for such powders.