The obesity and overweight growing rates are a major global concern for both developed and developing countries. Digital technologies can potentially deliver effective interventions to tackle this crisis, with mobile phones uniquely positioned to deliver scalable, real-time, inexpensive, and interactive support for all at risk. A meta-analysis showed that SMS interventions are effective for promoting weight loss and physical activity. The content of SMS messages of digital behaviour change interventions is usually seen as the main driver of effectiveness. It has been suggested that personalised communication, reaching people on emotional as well as rational levels, is more effective. However, there is no previous study exploring people’s affective responses to message wordings and personalisation, which could lead to more effective tools for promoting healthier behaviours, like physical activity. Affect is a term used to define the experience of a feeling or an emotion. Psychophysiological and self-reported measures of arousal and valence, the two main dimensions of emotions, were used to measure message impact on different levels of consciousness; and as methods to better understand people’s response to affective messages and personalisation. The first study in this thesis examined electrodermal activity and facial electromyography as objective measures of arousal and valence, respectively, and found strong associations between these measures. Study 2 compared psychophysiological and self-reported responses to previously evaluated affective and cognitive messages but found no significant difference between them. Study 3 used the same methods to test the effect of personalisation of affective and cognitive messages using identification (participant’s name) and contextualisation (participant’s preferred physical activity). Personalisation of messages using contextualisation and identification were found to be effective strategies for eliciting emotional responses and persuasiveness. This thesis contributes to our knowledge about using psychophysiology and self-report as methods to measure people’s response to affective and personalised messages, and the value of measuring the emotional impact of text messages on the recipient before using these in randomised controlled trials. The developers of SMS interventions and other digital techniques could benefit from using these methods. Future work needs to investigate the impact of messages designed using these methods on actual behaviour.