Pure methane plasmas have been produced in an inductively coupled radio frequency (ICRF) reactor with quartz walls in the absence of carbon film deposition. A differentially pumped mass spectrometer was used for the recording of the secondary hydrocarbon species produced in the reactor. The functional dependences of these species with plasma parameters has been analyzed and tentative mechanisms for their formation have been formulated. High power per molecule ratio promotes methane decomposition and acetylene formation.