Ion scattering using ultralow energy projectiles is considered to be a unique method to probe the nature of molecular surfaces because of its capacity to probe the very top, atomically thin layers. Here, we examine one of the most studied molecular solids, water-ice, using this technique. When ice surface undergoes the amorphous to crystalline transition, an ultralow energy reactive projectile identifies the change through the reaction product formed. It is shown that ultralow energy (2, 3, 4, 5, 6, and 7 eV) CH2+ (or CD2+) collision on amorphous D2O (or H 2O) ice makes CHD+, while crystalline ice does not. The projectile undergoes H/D exchange with the dangling -OD (-OH) bond present on amorphous ice surfaces. It is also shown that H/D exchange product disappears when amorphous ice is annealed to the crystalline phase. The H/D exchange reaction is shown to be sensitive only to the surface layers of ice as it disappears when the surface is covered with long chain alcohols like 1-pentanol as the ice surfaces become inaccessible for the incoming projectile. This article shows that ultralow energy reactive ion collision is a novel method to distinguish phase transitions in molecular solids. © 2013 American Chemical Society.