The present disclosure provides novel hemi- and mono-salts of 7-[(3R,4R)-3-Hydroxy-4-hydroxymethyl-pyrrolidin-1-ylmethyl]-3,5-dihydro-pyrrolo[3,2-d]pyrimidin-4-one (Compound 1) with various organic and inorganic acids. In one embodiment, the organic acid is a C4 organic diacids. The present disclosure further provides novel methods for preparing these salts. The novel monohydrates hemisalts of the C4 organic diacids are isostructural and can be prepared with different properties. Multiple acids can be used simultaneously and the proportion of acids can be varied offering the opportunity to select hemi-salts of compound 1 with desired properties.