Background. Antidepressant drug treatments increase the processing of positive compared to negative affective information early in treatment. Such effects have been hypothesized to play a key role in the development of later therapeutic responses to treatment. However, it is unknown whether these effects are a common mechanism of action for different treatment modalities. High-density negative ion (HDNI) treatment is an environmental manipulation that has efficacy in randomized clinical trials in seasonal affective disorder (SAD). Method. The current study investigated whether a single session of HDNI treatment could reverse negative affective biases seen in seasonal depression using a battery of emotional processing tasks in a double-blind, placebo-controlled randomized study. Results. Under placebo conditions, participants with seasonal mood disturbance showed reduced recognition of happy facial expressions, increased recognition memory for negative personality characteristics and increased vigilance to masked presentation of negative words in a dot-probe task compared to matched healthy controls. Negative ion treatment increased the recognition of positive compared to negative facial expression and improved vigilance to unmasked stimuli across participants with seasonal depression and healthy controls. Negative ion treatment also improved recognition memory for positive information in the SAD group alone. These effects were seen in the absence of changes in subjective state or mood. Conclusions. These results are consistent with the hypothesis that early change in emotional processing may be an important mechanism for treatment action in depression and suggest that these effects are also apparent with negative ion treatment in seasonal depression.