We have monitored the physical processes linked to electrochemical processes in situ occurring at MmNi(3.6)Mn(0.4)Al(0.3)Co(0.7) alloy negative electrodes using an acoustic emission (AE) technique in terms of waveforms, power spectra, and time history of the AE signals. Hydrogen evolution was characterized by an AE waveform with a long duration of over 0.15 ms and a power spectrum with a relatively narrow frequency distribution of around 0.6 MHz and a maximum amplitude at about 0.1 MHz. In contrast, cracking of the alloy particles was characterized by a burst-type AE waveform with duration shorter than 0.1 ms and large amplitude and a power spectrum with wide frequency distribution, with some peaks at frequencies more than 0.6 MHz. From the analyses of the AE waveforms, power spectra, and time history during the first charging process for the alloy negative electrode, it was inferred that cracking occurs due to lattice expansion by hydrogen absorption occurring intensively in the first half of the charging process. (c) 2006 The Electrochemical Society.