Diazines of the general formula (wherein X and Y stand for hydrogen or monovalent radicles which may be members of mono- or poly-nuclear carbocyclic or heterocyclic rings) are condensed with aldehydes or compounds splitting off aldehydes. The condensation may be effected in neutral, acid or alkaline medium, in the cold or hot, and in organic or inorganic liquids or melts. The condensation may be effected in the presence of fillers or dyestuffs or the products may be mixed with fillers or dyestuffs. Other compounds capable of condensation with aldehydes may be added to the condensation mixture. The resins may be employed in admixture with other natural or artificial resins such as phenol, urea, aniline, alkyd and ketone resins. The products which are resistant to heat and stable to light may be used as electric insulation, for moulded articles, and as lacquers or impregnating agents. In examples, 2 : 4 : 6-triaminopyrimidine or 2 : 4-diaminoquinazoline are condensed with formaldehyde. Reference has been directed by the Comptroller to Specification 455,008.