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Four deep eutectic solvents were used to produce plasticized thermoplastic starch films by a solution casting method. The mechanical properties of thin films, the stability of these properties over time, and the effects of humidity were investigated with the quartz crystal microbalance. The crystallinity, glass transition temperature, modulus and elongation at break for thicker films was measured using a combination of x-ray diffraction, differential scanning calorimetry and tensile testing. The films had very low glass transition temperatures (-50 ∘C or lower), with mechanical properties determined by the crystallinity of the films. The crystallinity did not change significantly over a period of one week, giving materials with a stable mechanical response. Deep eutectic solvents containing ammonium acetate, glycerol, and urea produced films with a crystalline structure that was much more stable at high humidity values than films plasticized only with glycerol or a traditional ionic liquid. These effects are attributed to the complex hydrogen bonding interactions between the different components of the deep eutectic solvents, and between these components and the starch polymer.