Thermal regenerators that use liquid heat transfer fluids are being researched for their application in active caloric cooling/heat pumping systems. The performance of active caloric devices is significantly influenced by the regenerator thermal effectiveness and by viscous losses. The present paper is the first part of a study on the thermal-hydraulic evaluation of packed bed regenerators that use water as a thermal fluid. Here, a detailed experimental analysis of the thermal-hydraulic performance of regenerative matrices composed of packed beds of stainless steel spheres is carried out. A laboratory apparatus was developed to quantify the viscous losses and thermal effectiveness in oscillating-flow regenerators that use water as the heat transfer fluid. Operating parameters such as frequency and mass flow rate were varied to cover broad ranges of utilization and number of transfer units (NM). The diameter of the spherical particles and the dimensions of the regenerator housing were also changed in order to evaluate their influence on the behavior of the time-dependent and time-average performance parameters. (C) 2016 Published by Elsevier Ltd.