The photoelectron spectrum of the trimethylenemethane (TMM) negative ion is described. The electron affinity of TMM is found from the spectrum to be 0.431 +/- 0.006 eV, and the energy difference between the X 3 A ' 2 and b 1 A 1 states of TMM is determined to be 16.1 +/- 0.2 kcal/mol. The energy difference between the lowest energy triplet and singlet states is estimated to be 13-16 kcal/mol. The enthalpy of formation of TMM is measured to be 70 +/- 3 kcal/mol, and the C-H bond enthalpy in 2-methylallyl radical is 90 +/- 2 kcal/mol. Previously unobserved vibrational frequencies of 425, 915, and 1310 cm - 1 are found for the triplet state of TMM, whereas a frequency of 325 cm - 1 is found for the singlet state. In addition, an overtone peak is observed for the triplet state at 1455 cm - 1 , and both states contain peaks that are assigned to bands arising from excited vibrational levels of the ion.