Evaluating the long‐term environmental effects of controlled‐availability nitrogen (N) fertilizers is best done with15N‐labeled materials. Simple bench‐scale methods not requiring elaborate apparatus are described for synthesizing15N‐enriched isobutylidene diurea (IBDU) and oxamide suitable for such purpose. IBDU is prepared by reacting15N‐labeled urea with isobutyr‐aldehyde. Oxamide is made by reacting15N‐labeled ammonia (NH3) with ethyl oxamate. No specific method but a general description is provided for synthesizing ureaform because the chemical composition of the family of polymers known as ureaform varies with reactant ratios and reaction conditions.