Formation of surficial sulfate– and halide–bearing salts by syn–eruptive ash–gas interactions is known to occur during volcanic eruptions. For reactions between aluminosilicates and the gas SO₂, at high temperature regimes (T≥ 600 °C), the controlling mechanism is the outward chemical diffusion of alkalis and alkaline earth metals, predominantly Ca²⁺, that result in sulfate salt formation, mostly CaSO₄, on glass surfaces. However, most of the experimental research has been conducted for SO₂ reactions with pure crystal–free, aluminosilicate glass, to simplify the complexities of crystal–bearing systems. Here, we tested high temperature SO₂–reactions using particles of a rhyolitic, crystal–bearing dome material from a 2013 eruption of Santiaguito volcano (Guatemala), by exposing 2 g of particles to 25 sccm of SO₂ at 600–800°C for 5–60 min each time. We then compare our results with those of previous studies using pure glass particles, aiming to determine the influence of crystal fraction and type on the occurrence and efficiency of gas–ash reactions. We conducted chemical and microscopic analysis of pre– and post–treated samples and observed that diffusion of Ca²⁺ is reduced in crystal–bearing samples relative to crystal–free samples at the same conditions. The rate of slow–down of the diffusion process appears to be dependent on the crystal volume fraction, providing a mechanism to account for this effect a priori. SEM images also showed that surface componentry strongly affects presence of CaSO₄, as salts appear to be absent on specific surface spots corresponding to crystal phases. Our results illustrate the need for ash-gas reaction studies to further consider both the effect of bulk– and surface–componentry, in order to more accurately assess syn-eruptive gas uptake by ash.