Performance differences in the growing number of electromagnetic (EM) sensors designed to estimate soil water content from a variety of indirect measurements (e.g., from measured travel time, capacitance, frequency shift) suggests the need for a standardized sensor characterization methodology. We suggest that characterization and evaluation of EM sensors, which currently lack citable standards, be performed in a homogeneous fluid of known permittivity rather than in a porous medium of unknown permittivity. Our objectives were to (i) develop a methodology for evaluating EM sensor measurement attributes referencing sensor-specific characteristics and targeted soil properties and (ii) suggest standards for characterization and comparison of sensors. Criteria for qualitative assessment of sensors include determination of effective measurement frequency; susceptibility to variations in salinity, dielectric relaxation, and temperature; and a look at spatial variation in sensor sampling area. Measurement frequencies for broadband sensors can be inferred from correlated network analyzer and sensor measurements or from manufacturer suggestions. Fluids were selected to provide surrogate soil-related effects such as relaxation occurring both within and outside of the effective measurement frequency range of common sensors. Test conditions included dielectrically relaxing (R) and nonrelaxing (NR) as well as electrically conducting (C) and nonconducting (NC) liquids and combinations thereof (e.g., NR-C). No suitable combination of relaxing and conducting (R-C) dielectric fluid was found in this study, but this remains a goal of future work because it represents a difficult and often common condition for EM sensor measurements in soils containing contributors to relaxation (e.g., clays, organic matter). Standards are based on fluids of known (based on Cole–Cole parameters) or measurable (using a network analyzer) frequency dependent permittivity that provide a reproducible homogeneous system for immersion of EM sensors. The methodology described here was applied using seven different EM sensing systems, and results are given in a companion paper.