Spin-lattice relaxation (SLR) of the photoexcited triplet state of tryptophan both as the free amino acid (Trp) and also incorporated at a buried site of ribonuclease T1 from Aspergillus oryzae (RNase T1) is investigated in the ethylene glycol (EG)-water mixed solvent system. Global analysis of microwave-induced delayed phosphorescence (MIDP) transients measured at 1.2 K is employed to obtain the triplet state kinetic parameters over a range of solvent composition between 10 and 90 vol % EG. These parameters are used to calculate the phosphorescence decay profile of the triplet states that contribute to the MIDP. The experimental phosphorescence profile is fitted at each solvent composition as the sum of the calculated nonexponential decay and a simple exponential decay with lifetime equal to the average tryptophan lifetime. The phosphorescence is modeled to originate from two distinct populations characterized by either slow SLR or rapid SLR relative to the triplet state lifetime. Only the former contributes to the MIDP signals while the latter is responsible for the exponential decay component. The fraction of tryptophan undergoing slow SLR, Phi(s), is at a maximum (ca. 0.7) for EG in the range 30-40% (v/v) for both Trp and RNase T1, diminishes sharply with increasing water content, and diminishes slightly with increasing EG content. The SLR of this population is independent of solvent composition. Since the tryptophan tripler state is not kinetically homogeneous in EG-water, the method of microwave-saturated phosphorescence decay analysis produces erroneous kinetic parameters for this system. The native RNase T1 structure becomes unstable above 60 vol % EG as revealed by changes in phosphorescence and ODMR.