The aim of this thesis is to determine the role of messages, adapted according to a user's personality traits, on the motivation of a mobile behaviour change application user. Positive behaviour change is a timely and relevant topic, as it is a key tool for reducing both the economic and health impact of preventable diseases, such as diabetes, obesity, and depression. To discover the impact of personalised content adaptation in behaviour change applications, we designed a study in which we created an Android application that allowed the user to set goals. We then tried to motivate the user with personalised messages. We analyzed the recorded data with statistical methods and confirmed that personalised messages are statistically significantly more motivating for some, while impersonalised messages remain more motivating for other users. This gives us confidence to further explore and identify characteristic groups susceptible to personalised persuasion based on personality, as well as those groups that are not susceptible to personalised persuasion.