This report is about an unexpected formation of N, N'-bis(substituted phenoxyacetyl) hydrazine derivatives. The reaction between N'-(4-ethylcyclohexylidene) phenoxyacetohydrazide with sulfanyl acids to obtain 4-thiazolidinones s, gave an unexpected product, N, N'-bis(phenoxyacetyl) hydrazine, with high purity instead of the expected cyclocondensation product. When repeating the reaction with p-toluenesulfonic acid under the same conditions, the reaction resulted in the same compound. This indicates that two molecules of N'-(4-ethylcyclohexylidene) phenoxyacetohydrazide were involved in the reaction. The reaction can be applied as a different and new procedure for the synthesis of several N, N'-bis(substituted phenoxyacetyl) hydrazine derivatives.