Methanol decomposition was carried out over flat cold-rolled foils of intermetallic compound Ni3Al in a temperature range 513–793 K. The methanol decomposition into H2 and CO was effectively catalyzed at 713–793 K, without any need for coating catalyst layers on the foil surface before reaction. Both catalytic activity and selectivity to H2 and CO production increased with time during the initial period of reaction, indicating that the Ni3Al foils were spontaneously activated under the reaction conditions. Analysis of the catalytic reaction revealed that the methanol decomposition accompanied three minor side reactions: the Boudouard reaction, (reverse) water–gas shift reaction, and methanation. Surface analyses revealed that fine Ni particles dispersed on carbon nanofibers formed on the Ni3Al foils during the reaction at 713–793 K. We attribute the high catalytic performance at 713–793 K to the gradual formation of this nanostructure.