A flowing-afterglow Langmuir probe apparatus with mass spectral analysis has been used to measure the rate constant for electron attachment to ClONO2 at 300 K. Electron attachment is efficient with a rate constant of 1.1 (+/-50%)x10(-7) cm(3) s(-1) and proceeds principally through dissociative channels to produce the major product ions NO2- (similar to 50%), NO3- (similar to 30%), and ClO- (similar to 20%). The parent ion ClONO2- and Cl- are also observed in the mass spectra but are at most minor products in the attachment process, less than or equal to 2% and less than or equal to 6%, respectively. A description of the secondary ion-molecule chemistry that takes place following electron attachment is given.