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Millimeter-wave observations of HCO+ have been conducted toward oxygen-rich circumstellar envelopes, as well as IRC + 10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 → 0 and 2 → 1 transitions of this molecule were measured with the ARO 12 m antenna, while the J = 3 → 2 and 4 → 3 lines were observed using the ARO Sub-Millimeter Telescope. HCO+ was detected toward the supergiant NML Cyg and the asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 → 1 and 3 → 2 lines of this ion were also detected toward IRC+ 10216, confirming the identification of HCO+ in this object. The line profiles measured for HCO+ toward NML Cyg consist of red- and blueshifted components, suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were f(HCO+/H2) ~ 0.15-1.3 x 10-7, with the AGB stars typically exhibiting the higher values. In IRC+ 10216, f(HCO+/H2) ~ 4.1 x 10-9, lower than the O-rich counterparts. The abundances of HCO+ were also found to peak at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H2O and CO, the main precursor species, suggests that HCO+ is more prevalent in envelopes that have substantial water, but CO also plays a role in its formation. The abundance of HCO+ appears to increase inversely with mass-loss rate, provided the rate is > 10-6 M⊙ yr-1. The common appearance of HCO+ in circumstellar gas indicates that, at some level, ion-molecule reactions influence the chemistry of evolved stellar envelopes. [ABSTRACT FROM AUTHOR]