Mobilities of polyatomic molecule ions in helium gas were measured at 85 K and room temperature using a low-temperature drift tube. They differed from the typical behavior shown by atomic or small molecular ions in helium owing to the influence of attractive interactions among the molecular ions and also their size and shape. To investigate the relationship between ion size and mobility, we measured the mobilities of CH3O+, C2H5O+, and C3H7O+. (C) 2017 Elsevier B.V. All rights reserved.