We prove a deviation bound for the maximum of partial sums of functions of α-dependent sequences as defined in Dedecker, Gouëzel and Merlevède (2010). As a consequence, we extend the Rosenthal inequality of Rio (2000) for α-mixing sequences in the sense of Rosenblatt (1956) to the larger class of α-dependent sequences. Starting from the deviation inequality, we obtain upper bounds for large deviations and an Hölderian invariance principle for the Donsker line. We illustrate our results through the example of intermittent maps of the interval, which are not α-mixing in the sense of Rosenblatt.