The activated carbons were synthesized from hay with CO2 as an activating agent. Furthermore hay and the chars obtained by pyrolysis were treated with urea, in order to introduce nitrogen to their structure, and then subjected to physical activation. The porous structure of activated carbons before and after impregnation was characterised by nitrogen adsorption using BET and BJH methods. The thermal properties of the samples were investigated by thermogravimetric (TG) analysis and differential thermogravimetry (DTG). The final products were activated carbons of low surface area varying from 84 to 196 m(2)/g and showing basic character of the surface. The impregnation of hay and chars with urea leads to a decrease in activated carbon surface area and pore volume. However, the materials modified by urea show a very high content of nitrogen (from 8.1 to 12.8 wt.%), which could be achieved thanks to the use of microwave heating as this method of heating permits a considerable reduction of the temperatures of pyrolysis and activation and a shortening of the time of activation. Following the character of TG and DTG curves, the nitrogen functional groups introduced into the activated carbon structure have relatively low thermal stability, as evidenced by the distinct loss of mass above the temperature of approximately 400 degrees C, observed both for unmodified carbon and the carbon enriched in nitrogen. (C) 2014 Elsevier B.V. All rights reserved.