Specific heats determined down to 0.3 K establish the existence of bulk superconductivity in a new a-phase superconducting salt, a-di[bis(ethylenedithio)tetrathiafulvalene]-ammoniumtetra(thiocyanato)mercurate [a-(ET}2{NH4)Hg(SCN)4] ( T, = 1 K), a derivative of the organic radical-cation donor molecule bis(ethylenedithio)tetrathiafulvalene (ET). This salt is important because it establishes a new structural class of organic superconductors, the a-phase class. The anomaly in the specific heat C is distinct but broad with a maximum in C/T at 0.8 K. The measurements yield a normal-state Sommerfield coefficient of y=29%2 mJ/K'mol, which is essentially identical to those of other ET-based organic superconductors, and an approximate value hC/y T, =1.1+0. 2, which is smaller than the BCS value of 1.43. A sharp but unexplained increase in C/T occurs below 0.7 K. Determinations of the temperature derivatives of the upper critical magnetic fields at T, ( -dH, 2/dT), from specific heats measured in applied magnetic fields, yield an extremely large value larger than 3 T/K for H~~ac, where ac denotes the crystallographic ac plane (which contains the tightly connected conducting layers of ET donor molecules), and a value of 0.1+0. 02 T/K for H//b '.

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