A new application of an ultra-fast thin film calorimeter is described. A commercially available sensor, thermal conductivity gauge TCG-3880 (Xensor Integrations, NL) was utilized as a measuring cell for isothermal calorimetric measurements. The sensor consists of a thin membrane (500 nm) with a semi conducting heater and a semi conducting thermopile, which are produced using integrated-circuit (IC) technology. To allow fast scanning sample size should be small too. Samples with masses of about 100 ng were measured. Different modes of operation were realized: (i) temperature scans over hundreds of Kelvin with rates up to 10,000 K/s; (ii) isothermal heat flow measurements with a time resolution of a few milliseconds; (iii) ac-mode for heat capacity measurements in the frequency range from 1 to 140 Hz at scanning or isothermally. In contrary to classical ac calorimeters beside heat capacity enthalpy changes can be monitored simultaneous. All these modes can be combined to realize experimental conditions needed to follow isothermal processes in time. As an example, we present isothermal crystallization experiments for fast crystallizing polymers. Heat flow as well as heat capacity was measured after quenching the sample from above the melting temperature to the crystallization temperature. Crystallization kinetics for polycaprolactone (PCL) and polyethylene (PE) are presented in a wide temperature range. We were able to follow isothermal crystallization of PCL in the whole temperature range between melting and glass transition. (C) 2003 Elsevier B.V. All rights reserved.