When a Ne:CH(3)X (X=F, Cl, Br) sample is codeposited at approximately 5 K with neon atoms that have been excited in a microwave discharge, the infrared spectrum of the resulting solid deposit includes prominent absorptions which can be assigned to cation products. It has previously been established that the ylidion (H(2)CXH(+)) isomers have stable potential minima and that H2CFH+ is lower in energy than CH3F+. The identification of the new absorptions is aided by experimental studies on isotopically substituted Ne:CH(3)X samples and by ab initio calculations of the structures and vibrational fundamentals of both CH(3)X(+) and H(2)CXH(+). In each of the three systems, a prominent absorption which is intermediate in frequency between the gas-phase (and neon-matrix) absorptions of HX and HX(+) can be assigned to the ylidion, as can be several other absorptions. Still other absorptions can be assigned to CH3Cl+ and CH3Br+. The behavior of the product absorptions on exposure of the deposit to filtered visible and ultraviolet radiation is consistent with the proposed assignments.