Losses of vitamins C and folates due to chemical degradation and leaching were modelled in green peas and Brussels sprouts. Vegetables were treated at four temperatures from 25 to 85 ◦C. They were directly placed in water (diffusion and thermal degradation) or hermetically sealed bags (only thermal degradation). Chemical degradation was modelled according to a pseudo-first-order and leaching according to Ficks’ second law. While in contact with water, losses of vitamins appeared to be ten times higher for vitamin C and two times for folates. In both vegetables, vitamin C diffused faster than folates, and the chemical degradation was faster in peas than in Brussels sprouts. For both vitamins, increasing of the temperature augmented the degradation speed and, to a lesser extent, the leaching. Chemical degradation of folates plateaued at approximately 53% and 63% of initial contents for pea and Brussels sprouts, respectively. The relative importance of degradation and leaching changed with vegetable size and temperature. In peas, leaching was the faster mechanism and notably predominant at low temperatures, probably due to the smaller size of the vegetable. An accurate understanding of vitamin loss requires simultaneous consideration of the two mechanisms studied here, degradation and leaching, which have different parameters.