The temperature-dependent EPR line shapes from the methyl rotor of the X irradiation-induced CH3C(COOH)(2) radical in powder MMA (methyl malonic acid) and the X irradiation-induced radical CD3C(COOH)(2) in methyl specifically deuterated powder MMA are studied experimentally for the temperature range 4.8 K (5 K) to 77 K (65 K). The hydrogenated system is simulated using a quantum inertial dynamical model with a hindering potential and three-site exchange rotation. The deuterated system is simulated using a classical three-site exchange model. The results show that due to the increase in moment of inertia, the tunneling frequency is negligible for the deuterated rotor, resulting in a stopped rotor low-temperature spectrum, while being sufficiently large for the hydrogenated system for this to exhibit tunneling, From the low-temperature deuterated analogue spectrum, the potential twist angle is estimated to delta = +/-50 degrees +/- 2 degrees (+n.60 degrees, n is an element of Z). The site-exchange activation energy of the deuterium rotor is observed to be 387 K, substantially lower than the hindering potential depth of 618 K, The hydrogenated system exchange rotation rate assumes a linear behavior with 754 K activation energy in the classical region of temperatures above 50 K.