We investigate differences in the psychological aspects underpinning Western mobilisation of two terrorist groups by analysing their English-language propaganda. Based on a computerised analysis of the language used in two English-language online magazines circulated by Islamic State of Iraq and Syria (ISIS) and al-Qaeda (i.e., Dabiq and Inspire), we found significant differences in their language—the ISIS’ language being higher in authoritarianism and its level of religiousness. In a follow-up experimental study, we found that being high in religiousness and authoritarianism predicts more positive attitudes towards the language used by ISIS, but not towards the language used by al-Qaeda. The results suggest that ISIS’ propaganda may be more effective in mobilising individuals who are more authoritarian and more focused on religion than that of al-Qaeda. These findings are consistent with the behaviour observed in recent homegrown terrorist attacks in the United States and Europe.