The aim of this study was to investigate whether language-specific properties influence mental number processing. German Sign Language (DGS) numbers differ from those in spoken German not only in terms of modality but also in their basic language structure. A group of 20 congenitally deaf German signers participated in a number parity (odd/even) judgment task with DGS and printed German number words. The results indicated that two-handed DGS number signs are processed in a decomposed way. This language-specific effect also generalized to another linguistic number notation, German number words, but not to Arabic digit notation. These differences are discussed with respect to two possible routes to number parity.