Dynamic nuclear polarization (DNP) at 3.35 T and 1.2 K is characterized for [1-13C]pyruvic acid and the paramagnetic agent OX063Me. This paramagnetic agent belongs to a class of organic radicals (trityls) with unique properties for DNP. [1-13C]Pyruvic acid is a molecule of particular relevance for in vivo metabolic studies, and was chosen for that reason. The studies in this work support the conclusion that DNP is through the thermal mixing effect. Critical parameters for DNP are studied: the electron spin resonance spectrum and relaxation times, the nuclear relaxation time and the microwave frequency dependence of the DNP enhancement. In particular, it is reported that the addition of small quantities of Gd3+ affects the DNP favorably. Of the studied parameters only the trityl electron longitudinal relaxation time is significantly affected by the presence of Gd3+. The current models for DNP by the thermal mixing effect, the high-temperature Provotorov equations and the low-temperature Borghini model, are unable to quantitatively explain the observed DNP.