The invention relates to a valve for opening and closing a channel (3) of a microfluidic system, respectively. According to the invention, the valve comprises an actuation medium (2) that undergoes a volume change with changing temperature; and a heater arrangement (5) for generating a temperature gradient in the actuation medium (2) with respect to the actuation medium's (2) distance relative to the channel (3); wherein due to an expansion or a contraction of the actuation medium (2) the channel (3) is closed or opened, respectively. When the heater arrangement (5) is activated in such a way that a higher temperature is generated in the actuation medium (2) which is nearer to the channel (3) and a lower temperature is generated in the actuation medium (2) which is further away from the channel (3) the valve can be closed and vice versa. Accordingly, such a valve for a microfluidic system is provided which can be reliably actuated during a long time of use.