An analytical moisture diffusion model which considers the influence of external resistance to mass transfer is developed. The methodology to determine constant and variable moisture diffusion coefficients, Deff is proposed. A laboratory model of mixed-mode solar dryer is constructed to perform 16 experiments for different performance dependent variables under simulated indoor conditions. The potatoes (Solanum tuberosum) of Kufri Safed variety have been chosen as the test food product. The range of variables investigated is absorbed thermal energy (150-750 W/m2); air mass flow rate (0.009-0.022 kg/s); loading density (1.08-4.33 kg/m2) and sample thickness (5-18 mm). The efficiency results have been analysed to identify the value of each process variable leading to optimal operation of dryer. The study reveals that dryer with sample thickness of 8 mm and loading density of 4.33 kg/m2 can operate optimally for absorbed energy of 450 W/m2 and air mass flow rate of 0.017 kg/s. © 2013 © 2013 Taylor & Francis.