[Show abstract][Hide abstract] ABSTRACT: We have carried out experimental and theoretical studies of Penning ionization processes occurring in thermal energy collisions of state-selected metastable He*(23S) and He*(21S) atoms with ground state alkaline earth atoms X(X=Mg, Ca, Sr, Ba). Penning ionization electron energy spectra for these eight systems, measured with a crossed-beam set-up perpendicular to the collision velocity at energy resolutions 40-70rmeV, are reported; relative populations of the different ionic X+ (m’) states are presented and well depths D*e for the He*+X entrance channel potentials with uncertainties around 25rmeV are derived from the electron spectra as follows: He*(23S)+Mg/Ca/Sr/Ba: 130/250/240/260rmeV; He*(21S) +Mg/Ca/Sr/Ba: 300/570/550/670rmeV. The spectra show substantial differences for the three ionic states X+(2S), X+(2P) and X+(2D) and reveal that transitions to a repulsive potential - attributed to He+X+(2P) 2 formation - are mainly involved for the X+(2P) channel. Ab initio calculations of potential curves, autoionization widths, electron energy spectra and ionization cross sections are reported for the systems He*(23S)+Ca and He*(21S)+Ca. The respective well depths D*e are calculated to be 243(15)rmeV and 544(15)rmeV; the ionization cross sections at the experimental mean energy of 72rmeV amount to 101rư and 201rư, respectively. Very good overall agreement with the experimental electron spectra is observed.