The invention is related to control of transmissions in spread spectrum radio systems, more accurately to estimating transmission power increases caused by new transactions in the system. According to the invention, the estimate or interference power increase due to a new transaction is calculated at least partly on the basis of the current fractional load, the current received interference power level, and a load factor L, which is calculated essentially on the basis of the chiprate, the bitrate of the new transaction, and the estimated required signal-to-interference ratio for the service type of the new transaction.