The molecular and crystal structure of 3-nitropyrazole was determined by X-ray analysis. The triclinic unit cell contains 12 molecules which form four hydrogen-bonded (N—H···N) trimers. Each trimer comprises of pseudo-ring in a flattened envelope distorted towards a chair conformation. The crystal packing consists of layers formed by centrosymmetric related trimers joined through C—H···O interactions. Ab initio calculations were performed on 3(5)-nitro- and 4-nitropyrazole and their corresponding protonated forms up to the MP2/6–31G** level of theory. The origin of the difference in aqueous basicities between both nitropyrazoles is discussed. © 1997 John Wiley & Sons, Ltd.