Provided are novel crystalline forms of a spiro-compound which acts as a muscarinic acetylcholine receptor agonist. In particular, isolated crystalline polymorphs of (S)-2-ethyl-8-methyl-1-thia-4,8-diazaspiro[4.5]decane-3-one are described which have favorable properties in pharmaceutical manufacture. Also provided are methods to prepare said crystalline polymorphs, and to convert them into each other as well as methods for preparing medicaments containing the same which are suitable for use in the treatment of diseases and disorders that respond to modulation of the muscarinic acetylcholine receptor.