Tris and methanol have been used quite extensively as probes for surface-bound hydroxyl radicals in heterogeneous systems, such as TiO2 photocatalysis and catalytic decomposition of H2O2. Recent studies have indicated that the selectivity for surface-bound hydroxyl radicals is questionable and that the yield of the stable detectable product, formaldehyde, may be different for different reactive species. In this work, we have explored the selectivity as well as the formaldehyde yield of these two probes by experimentally studying formaldehyde formation in homogeneous (gamma radiolysis) and heterogeneous (TiO2 photocatalysis) systems, where hydroxyl radicals and Br-2.- can be formed. The latter is formed in the reaction between hydroxyl radicals and Br- at relatively high concentrations of Br-. The experiments clearly show that the formaldehyde yield is reduced by 85% when comparing hydroxyl radicals and Br2 & BULL;- in a homogeneous methanol system and increased by 50% when making the same comparison for the homogeneous Tris system. The increased yield is attributed to a change in the reaction mechanism as Br2 & BULL;- is mainly expected to produce a nitrogen-centered radical cation, while the hydroxyl radical mainly produces carbon-centered radicals. The radical cation appears to produce formaldehyde with a higher yield. In the photocatalysis system, the trends are similar but even more emphasized. The rationale for this is discussed, as well as the suitability of methanol and Tris as probes for surface-bound hydroxyl radicals in heterogeneous systems.