The expression of O-acetylated sialic acids in human colonic mucins is developmentally regulated, and a reduction of O-acetylation has been found to be associated with the early stages of colorectal cancer. Despite this, however, little is known about the enzymatic process of sialic acid O-acetylation in human colonic mucosa. Recently, we have reported on a human colon sialate 7(9)-O-acetyltransferase capable of incorporating acetyl groups into sialic acids at the nucleotidesugar level [Shen et al., Biol. Chem. 383 (2002), 307317]. In this report, we show that the CMP-N-acetyl-neuraminic acid (CMPNeu5Ac) and acetyl-CoA (AcCoA) transporters are critical components for the O-acetylation of CMPNeu5Ac in Golgi lumen, with specific inhibition of either transporter leading to a reduction in the formation of CMP-5-Nacetyl-9-O-acetylneuraminic acid (CMP-Neu5,9Ac2). Moreover, the finding that 5-Nacetyl-9-O-acetylneuraminic acid (Neu5,9Ac2) could be transferred from neo-synthesised CMP-Neu5,9Ac2 to endogenous glycoproteins in the same Golgi vesicles, together with the observation that asialofetuin and asialo-human colon mucin are much better acceptors for Neu5,9Ac2 than asialo-bovine submandibular gland mucin, suggests that a sialyltransferase exists that preferentially utilises CMPNeu5,9Ac2 as the donor substrate, transferring Neu5,9Ac2 to terminal Galβ1,3(4)R- residues.