In Ohno’s classic theory for the evolution of dosage compensation, as Y gene function is lost, gene expression on the X chromosome is upregulated to restore ancestral autosomal expression levels in males. This upregulation of X genes carried through to females, who then evolved X chromosome inactivation to transcriptionally silence one X. This hypothesis is challenged by observations in platypus (and other species) where expression from the X’s in males is lower than from the autosomes, and from the X’s in females. However, all previous large scale studies of dosage compensation have focused only on total mRNA levels, and have ignored the possibility that gene dosage could change post-transcription. In this study we analyse dosage compensation of opossum and platypus X genes at three different stages from the genome to the proteome: 1) total mRNA; 2) mRNAs bound to ribosomes; 3) total protein abundance. As expected, we observed that total mRNA levels of X genes in male platypus were less than 2/3 (0.62) that observed in females. In contrast, mRNA bound to ribosome in males was 3/4 (0.74) of that observed in females. Finally, we observed that protein levels were much closer to balanced between sexes (0.88). These results demonstrate that a lack of global dosage compensation in the transcriptome does not necessarily carry through to the functional unit. This is the first example that dosage compensation of X genes can be corrected post transcriptionally in mammals, and has large implications for inconsistencies observed with dosage compensation in other species.