Soft-tissue sarcomas are often highly heterogeneous tumours. Clinical trials of non-surgical treatments, for example combined radiotherapy and systemic agents, require non-invasive methods for response assessment. Quantitative MRI provides non-invasive response assessment of the whole tumour volume, but metrics require validation against histopathology. In this study, 26 patients with retroperitoneal sarcoma were imaged prior to surgery, with written consent, as part of a prospective single-centre study. Diffusion-weighted MRI parameters (ADC, and D from IVIM) showed correlation with nuclear-to-stromal ratio, and were also related to stroma type and stroma grade. Dixon-derived fat fraction correlated strongly with histopathological assessment of fat fraction.