Transitions from the L-mode regime to the I-mode regime, with an energy transport barrier, and to the H-mode regime with both an energy and particle transport barrier are studied on the Alcator C-Mod tokamak. Steady I-mode plasmas have been produced over a wide range of plasma field (3–6 T), current (0.8–1.35 MA), density and shaping in the unfavourable ion B × ∇B configuration. The power threshold for the L–I transition is higher than scalings for the L–H transition with favourable drift, and increases with plasma current as well as density. Threshold conditions for the I–H transition are more variable. In some conditions I-mode is maintained up to the maximum available ICRF power of 5 MW, nearly a factor of two above the L–I threshold, giving a robust operating window. Edge Te at the L–I transition is in the range 250–450 eV, over a range of current and density, about a factor of two higher than with favourable drift, while at the I–H transition it can be much higher (up to 1.1 keV) but is again widely variable. Heat pulses due to sawteeth may play a role in transitions. Controlled I–L back transitions indicate little power hysteresis.