Cyprodinil is an anilinopyrimidine folar fungicide which is believed to act by inhibiting the biosynthesis of methionine and the secretion of fungal hydrolytic enzymes. It is widely used in the EU on cereals, grapes, pome fruits, strawberries and in barley seed dressings. Two polymorphic forms are known with different melting points, A = 71°C and B = 75°C; phase transition between the two forms can occur between 15 and 40 °C with A stable at the lower temperatures. This polymorphism is related to a change in the hydrogen bonding between the molecules from a moderately strong H-bonded ( N-H....N(aromatic ring)) dimeric ring (in B form) to a weaker single interaction in the A form. This behaviour is highly important to the commercial use of this material as one form undergoes a slow conversion to the other while in storage causing particle aggregation. We need to define with high accuracy the hydrogen bonds and other weaker intermolecular interactions in the two polymorphs of cyprodinil in order to understand the phase transition behaviour and use this information in modelling the formation of new polymorphs and co-crystals.