Univariate sensitivity analysis was used to determine parameter importance within the model of gas exchange (MOXE) for lung hyperpolarised 129Xe MRS/MRI. Sensitivity of the model to parameter variation across a range of saturation recovery delay times within the chemical shift saturation recovery (CSSR) technique was assessed. Simulated parameter values spanned those in healthy and patient groups. MOXE is found to be most sensitive to changes in surface-area-to-volume ratio and in septal wall thickness. MOXE is negligibly sensitive to alveoli-capillary barrier thickness. CSSR measurements taken within delay periods between 0 - 200 ms provide optimum parameter sensitivity.