Microwave thermal ablation is under consideration as a minimally invasive modality to treat 10-20 mm benign adrenal adenomas, while preserving normally functioning adjacent adrenal tissue, and returning the gland to a normally functioning status that is under normal regulation. In contrast to applications for tumor ablation, where devices have been developed with the objective of maximizing the size of the ablation zone for treating large tumors, a challenge for adrenal ablation is to minimize thermal damage to non-targeted adrenal tissue and thereby preserve adrenal function. Here, we investigate methods for creating small spherical ablation zones of volumes in the range 0.5-4 cm(3) for the treatment of benign adrenal adenomas using water-loaded microwave monopole antennas operating at 2.45 GHz and 5.8 GHz. Coupled electromagnetic and bioheat transfer simulations and experiments in ex vivo tissue were employed to investigate the effect of frequency, applied power, ablation duration, and coolant temperature on the length and width of the ablation zone. Experimental results showed that small spherical ablation zones with diameters in the range of 7.4-17.6 mm can be obtained by adjusting the applied power and ablation duration. Multi-way ANOVA analysis of the experimentally-measured ablation zone dimensions demonstrated that frequency of operation and ablation duration are the primary parameters for controlling the ablation zone length and width, respectively. Additionally, it was demonstrated that the coolant temperature provides another effective parameter for controlling the ablation zone length without affecting the ablation zone width. Our study demonstrates the feasibility of creating small spherical microwave ablation zones suitable for targeting benign adrenal adenomas.