The invention discloses a gas-turbine plant provided with thermoacoustic instability detection and a method of controlling a gas-turbine plant. The gas-turbine plant includes a gas-turbine unit and a control system (3). The gas-turbine unit includes a combustion chamber (5). The control system includes monitoring apparatus (9), which is provided with at least one optical probe (11) and configured to detect light intensity fluctuations inside the combustion chamber by the optical probe, and a controller (10), configured to control the gas-turbine unit according to a response of the monitoring apparatus.