A method for cleaning an inline optical sensor in a system which comprises: a flowcell comprising a chamber with an inlet and an outlet ; one or more flushing channels which debouch in the chamber; wherein the sensor comprises one or more substantially transparent windows arranged in the chamber , the sensor being arranged for sensing an optical property of a liquid flowing through the chamber, the one or more substantially transparent windows being arranged for spacing a light source and/or a light detector of the sensor apart from the liquid flowing through the flowcell ; characterized in that the method comprises the steps of: blocking flow of liquid through the inlet into the chamber ; supplying air pressurized at a first air pressure (P1) to the chamber for substantially emptying the chamber from liquid; and dispensing an amount of cleaning liquid, and using air pressurized at a second air pressure (P2), vaporizing said amount of cleaning liquid to obtain a cleaning liquid/pressurized air mixture and delivering said mixture via the one or more flushing channels onto the one or more windows in the chamber; wherein the second air pressure (P2) is greater than ambient pressure and at least two times smaller than the first air pressure (P1); letting the cleaning liquid from the mixture act on the one or more windows onto which the mixture has been delivered, for at least a predetermined amount of time; and flushing the chamber with hot water which has a temperature of at least 80 degrees Celsius. a flowcell comprising a chamber with an inlet and an outlet ; one or more flushing channels which debouch in the chamber; wherein the sensor comprises one or more substantially transparent windows arranged in the chamber , the sensor being arranged for sensing an optical property of a liquid flowing through the chamber, the one or more substantially transparent windows being arranged for spacing a light source and/or a light detector of the sensor apart from the liquid flowing through the flowcell ; characterized in that the method comprises the steps of: blocking flow of liquid through the inlet into the chamber ; supplying air pressurized at a first air pressure (P1) to the chamber for substantially emptying the chamber from liquid; and dispensing an amount of cleaning liquid, and using air pressurized at a second air pressure (P2), vaporizing said amount of cleaning liquid to obtain a cleaning liquid/pressurized air mixture and delivering said mixture via the one or more flushing channels onto the one or more windows in the chamber; wherein the second air pressure (P2) is greater than ambient pressure and at least two times smaller than the first air pressure (P1); letting the cleaning liquid from the mixture act on the one or more windows onto which the mixture has been delivered, for at least a predetermined amount of time; and flushing the chamber with hot water which has a temperature of at least 80 degrees Celsius. blocking flow of liquid through the inlet into the chamber ; supplying air pressurized at a first air pressure (P1) to the chamber for substantially emptying the chamber from liquid; and dispensing an amount of cleaning liquid, and using air pressurized at a second air pressure (P2), vaporizing said amount of cleaning liquid to obtain a cleaning liquid/pressurized air mixture and delivering said mixture via the one or more flushing channels onto the one or more windows in the chamber; wherein the second air pressure (P2) is greater than ambient pressure and at least two times smaller than the first air pressure (P1); letting the cleaning liquid from the mixture act on the one or more windows onto which the mixture has been delivered, for at least a predetermined amount of time; and flushing the chamber with hot water which has a temperature of at least 80 degrees Celsius. <EOS>