One increasing trend on social media is the posting of body positive content that aims to challenge narrow beauty ideals and instead promote acceptance and appreciation of all bodies. The aim of the present study was to experimentally investigate the effect of body positive captions attached to Instagram images on young women's body image. Participants were 384 women aged 18-30 years randomly assigned to view Instagram images of thin or average-sized women containing either body positive captions or no captions. In contrast to prediction, the body positive captions had no effect on body dissatisfaction or body appreciation. There was a significant effect of image type, whereby the average images resulted in less body dissatisfaction and greater body appreciation than the thin images. A significant three-way interaction indicated that for women high on thin-ideal internalisation, body positive captions on average images led to greater body appreciation, but lower body appreciation when attached to thin images. The results suggest that the visual imagery of an Instagram post is a more potent contributor to body image than any accompanying text. Presenting a more diverse array of women's bodies on social media is likely a more effective way to foster body satisfaction and appreciation.