The nature of the N- and O- linked glycosylation of acetylxylan esterase (AXE) of the Trichoderma reesei strain Rut-C30 has been characterized using different enzymatic, chromatographic, and mass spectrometric techniques. The combined data showed that the AXE N-glycan is phosphorylated and highly mannosylated. The predominant N-glycans on the single glycosylation site on AXE can be represented as GlcNAc(2)Man((1-6))P. The linker-substrate binding domain peptide separated from the core by papain digestion is heavily O-glycosylated and consists of mannose, galactose, and possibly glucose as monosaccharide and disaccharide substituents. In addition to glycosylation, sulfation was observed in the linker region. Both N- and O- linked glycans show remarkable heterogeneity. Three isoforms of AXE, separated by 2D SDS-PAGE, are described with pI values of 5.0, 5.3, and 5.9. The three isoforms can be explained by posttranslational modification of the enzyme by glycans, phosphate, and sulfate. Advancing the knowledge on the nature of the glycans produced by T. reesei is elementary for its use as a host for the expression of heterologous glycoproteins of industrial and pharmaceutical importance.