The present invention provides co-crystals of glycopyrronium bromide with lactose. The glycopyrronium bromide and lactose in the novel co-crystals are present in a stoichiometric ratio of from about 1:2 to 2:1. These are characterized by XRD and DSC. Processes for preparing the novel co-crystals are also provided. The co-crystals are also disclosed for use as a medicament, in particular, for treatment of respiratory complaints, such as chronic pulmonary obstructive disease (COPD), bronchitis and asthma. Pharmaceutical compositions comprising the co-crystals as active ingredient are also presented.