Endoplasmic reticulum (ER)‐to‐Golgi transport is blocked in mammalian cells during mitosis; however, the mechanism underlying this blockade remains unknown. Since COPII proteins are involved in this transport pathway, we investigated at the biochemical level post‐translational modifications of COPII components during the course of mitosis that could be linked to inhibition of ER‐to‐Golgi transport. By comparing biochemical properties of cytosolic COPII components during interphase and mitosis, we found that Sec24p isoforms underwent post‐translational modifications resulting in an increase in their apparent molecular weight. No such modification was observed for the other COPII components Sec23p, Sec13p, Sec31p or Sar1p. Analyzing in more details Sec24p isoforms in interphase and mitotic conditions, we found that the interphase form of Sec24p was O‐N‐acetylglucosamine modified, a feature lost upon entering into mitosis. This mitotic deglycosylation was coupled to Sec24p phosphorylation, a feature likely responsible for the increase in apparent molecular weight of these molecules. These modifications correlated with an alteration in the membrane binding properties of Sec24p. These data suggest that when entering into mitosis, the COPII component Sec24p is simultaneously deglycosylated and phosphorylated, a process which may contribute to the observed mitotic ER‐to‐Golgi traffic block.