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Interest in the observation and characterization of organic isomers in astronomical environments has grown rapidly with an increase in the sensitivity of detection techniques. Accurate modeling and interpretation of these environments require experimental isomer-specific reactivity and spectroscopic measurements. Given the abundance of formaldehyde (H 2 CO) in various astrophysical objects, the properties and reactivities of its cation isomers H 2 CO •+ and HCOH •+ are of significant interest. However, for the hydroxymethylene radical cation HCOH •+ (and its isotopologue DCOH •+ ), detailed reactivity studies have been limited by the lack of suitable experimental methods to generate this isomer with high purity. Here, potential approaches to the isomer-selective generation of HCOH •+ and DCOH •+ are characterized through differential reactivity measurements. While the dissociative photoionization of cyclopropanol (c-CH 2 CH 2 CHOH) is determined to be unsuitable, the dissociation of methanol- d 3 (CD 3 OH) allows for the formation of DCOH •+ with a fractional abundance of >99% at photon energies below 14.8 eV. These results will allow future spectroscopic and reactivity measurements of HCOH •+ /DCOH •+ to be conducted, laying the groundwork for future detection and incorporation into models of the interstellar medium.