The first measurements of toroidal rotation of the bulk plasma during purely ion cyclotron resonance frequency (ICRF) heated H-modes on the JET tokamak are reported. Substantial toroidal acceleration in the direction of the plasma current occurs at the transition into H-mode. In the present paper the relationship between toroidal rotation and improved confinement of the H-mode is investigated. We conclude that the dominant driving mechanism for toroidal rotation is the ion pressure gradient.