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We have measured the 2/> -Is 7r-mesonic and muonic x-ray energies and widths for Z (atomic number) =3 (lithium) through 12 (magnesium) in order to study the effect of the nucleus on 7r-mesonic x-ray energy levels. The search for the tf-mesonic 2p -Is x ray in higher Z elements is difficult because of the low yields and large natural linewidths of the x rays. As Z increases, the yield decreases because of nuclear capture from the 2p state, and the x-ray line is broadened because of fast nuclear capture from the Is state.Our use of solid-state germanium and silicon detectors allowed the measurement of xray energies with high resolution. Optimum resolution was obtained by using both silicon and germanium detectors to exploit the relative advantages of each detector. The silicon detector had a better resolution (1.2 keV) than germanium (2.5 keV), but it had a lower efficiency for energies above 70 keV. Electronic shifts in gain were minimized by using two digital gain stabilizers to stabilize the lower and upper end of the energy window. All targets were at least 95% isotopically pure except magnesium, for which we used the naturally occurring element, and B 10