A method for cleaning process gases, such as flue gases formed during combustion in a boiler, especially during combustion of waste, in which method a sorbent, such as an absorbent and/or an adsorbent, is caused to react with gaseous pollutants present in the process gases, such as acidifying components and/or heavy metals, and/or organic compounds, thereby forming particulate pollutants, which are separated in a subsequent dust separator, such as a bag filter (6). In the method, the moisture content and temperature of the process gases as well as the content of at least one of the gaseous pollutants are measured. The relative humidity of the process gases is increased from a substantially constant basic level (BL) to an increased level (BI), this increase (I) being initiated by said content exceeding an upper limit value (UL). The amount of sorbent supplied to the process gases is then increased. The relative humidity of the process gases is decreased after a limited first period (t51), and/or this decrease is initiated by said content being below a lower limit value (LL), thereby restoring the relative humidity to said basic level (BL).