The dynamic nuclear polarization effect between the nitroxide radical TEMPO and the ¹H protons of water solution was investigated at an electron pumping frequency of 9.7 GHz for different experimental conditions. In particular, we compared ¹⁴N-4-hydroxy-TEMPO (TEMPOL) with 4-oxo-TEMPO and evaluated the effect of ²H and ¹⁵N isotope labelling. Furthermore, we tested the effect of concomitant irradiation on both electron paramagnetic resonance (EPR) lines with the ¹⁵N-labelled compound. Our results show that the reduction in the EPR line width given by the ²H and ¹⁵N labelling and the collapse in the number of hyperfine lines due to ¹⁵N substitution leads to the highest enhancement of ε = −140 ever reported in the literature. Concomitant pumping on two hyperfine lines does not give higher enhancements at our experimental conditions.