The internet seems promising for delivering interventions to enhance well-being in a normal population. The aim of this study was to test the effects of an internet-based positive psychology intervention targeting gratitude pleasant activities strengths mastery acts of kindness optimism flow attributions and mindfulness. One-hundred and twelve participants were randomized to the intervention and 94 to the control group and data were collected at baseline one two and six months after intervention onset. The balance of positive to negative affect increased over time among participants in the intervention group (unstandardized beta coefficient [B] = 0.07 p < 0.01) as compared to the controls (B = 0.02 p = 010). The intervention worked equally well regardless of participants’ gender age or education. However optimism did not mediate the effect of the intervention on affect balance. In conclusion the intervention had a small but significant effect on affect balance among healthy adults.