The concurrent propagation of the aromatics-based and olefins-based catalytic cycles at early stages of the methanol-to-olefins reaction over HSAPO-34 and the resulting consequences on light olefins selectivities are demonstrated with C-13(3)-propylene/C-12(2)-dimethyl ether isotopic tracing studies at 6231( and sub-complete dimethyl ether conversions. Transients in effluent product selectivities were rationalized by the maturation of the entrained hydrocarbon pool where catalyst turnover number is introduced as a compendious descriptor of hydrocarbon pool maturity and reaction progress. The distinct C-13-content of ethylene from other effluent products and its agreement with the C-13-content of entrained polymethylbenzenes indicate that ethylene is a product of aromatics-based dealkylation events while the match between methylation-predicted and experimentally observed C-13-contents for C5+ olefins establishes that they are products of olefins-based methylation events. Methanol-to-olefins conversion proceeds through a dual cycle mechanism proposed earlier for methanol conversion over other solid acid catalysts where the topology of HSAPO-34 specifically engenders the prevalence of the aromatics-based cycle at >similar to 200 mol(c) mol(H+)(-1) catalyst turnovers. (C) 2016 Elsevier Inc. All rights reserved.