Micromachined Joule-Thomson (JT) coolers can be used for cooling small electronic devices. For this application, two types of micromachined JT cold stages with dimensions of 60.0 × 10.0 × 0.7 mm3 were developed and tested that were designed for operation with nitrogen at 100 K. A theoretical analysis is developed to investigate the application of these cold stages in the temperature range 80–250 K. This analysis shows that the cold stages can be operated with various working fluids. Experiments of both JT cold stages operating with nitrogen and methane as working fluids were done to validate this analysis. The cooling power and the temperature profile along the length of the counter-flow heat exchanger were measured. In this paper, the theoretical analysis is described and the measuring results are presented and discussed.