A mechanism for an office chair having a backrest support which may be pivoted rearwardly. Adjustment of the backrest pivotal motion is accomplished by the mechanism having a base support which may be positioned on a chair column, a seat support, the backrest support which may be pivoted to the rear, and a pre-tensioned spring arrangement for enabling the mechanism to counter or oppose the movement of the backrest support. The seat support and the base support form a movable unit, which may be moved relative to the chair column depending on the weight of a user applying a load to the seat support. Movement of the movable unit causes an adjustment of the pre-tension of the spring arrangement and/or an adjustment of the spring constant of the spring arrangement.