A sequence of phenol-formaldehyde and m-cresol-formaldehyde orthonovolac resins were synthesized by varying the mole ratios of phenol or substituted phenol to formaldehyde. The resins were characterized by their thermal properties such as softening temperatures and degradation profile. The non-isothermal procedures of Coats-Redfern and Horowitz-Metzger were applied to their respective differential thermogravimetric plots to ascertain the thermal degradation kinetics of these resins in air. The thermal stability and activation energies (E) of m-cresol-formaldehyde resins in the terminal degradation stage were correlated with those of the phenol-formaldehyde resins. The effect of aromatic ring substitution on the thermal stability is also discussed.