A scroll vacuum pump comprising: an orbiting scroll having an orbiting scroll wall extending axially from an orbiting scroll base towards a fixed scroll, the fixed scroll having a fixed scroll wall extending axially from a fixed scroll base towards the orbiting scroll; an axially extending drive shaft having an eccentric shaft portion so that rotation of the eccentric shaft portion imparts an orbiting motion to the orbiting scroll relative to the fixed scroll; and an axial thrust bearing arrangement comprising: an array of ball bearings for bearing against the orbiting scroll base in an axial direction, each ball bearing describing a circular path over the orbiting scroll base during orbiting motion; at least one thrust surface for bearing against the array of ball bearings; a retainer for retaining the ball bearings in location relative to said at least one thrust surface and the orbiting scroll; and an adjustment mechanism comprising a plurality of adjustment members angularly spaced apart about the axis of the pump operable for adjusting the axial position of said at least one thrust surface and thereby the axial position of the orbiting scroll base, wherein the adjustment members are threaded and co-operate with threaded bores in the retainer such that rotation of the adjustment members causes adjustment of the axial position and/or angle of the retainer; characterized in that the scroll pump further comprises a plurality of biasing members configured to bias the adjustment mechanism relative to the thrust surface or surfaces for resisting relative movement between the thrust surface or surfaces and the adjustment mechanism; said plurality of biasing members being for respective adjustment members for causing the threads of the adjustment members to bear against the threads of the bores in the retainer for resisting play between the threads and/or increasing friction therebetween. an orbiting scroll having an orbiting scroll wall extending axially from an orbiting scroll base towards a fixed scroll, the fixed scroll having a fixed scroll wall extending axially from a fixed scroll base towards the orbiting scroll; an axially extending drive shaft having an eccentric shaft portion so that rotation of the eccentric shaft portion imparts an orbiting motion to the orbiting scroll relative to the fixed scroll; and an axial thrust bearing arrangement comprising: an array of ball bearings for bearing against the orbiting scroll base in an axial direction, each ball bearing describing a circular path over the orbiting scroll base during orbiting motion; at least one thrust surface for bearing against the array of ball bearings; a retainer for retaining the ball bearings in location relative to said at least one thrust surface and the orbiting scroll; and an adjustment mechanism comprising a plurality of adjustment members angularly spaced apart about the axis of the pump operable for adjusting the axial position of said at least one thrust surface and thereby the axial position of the orbiting scroll base, wherein the adjustment members are threaded and co-operate with threaded bores in the retainer such that rotation of the adjustment members causes adjustment of the axial position and/or angle of the retainer; characterized in that the scroll pump further comprises a plurality of biasing members configured to bias the adjustment mechanism relative to the thrust surface or surfaces for resisting relative movement between the thrust surface or surfaces and the adjustment mechanism; said plurality of biasing members being for respective adjustment members for causing the threads of the adjustment members to bear against the threads of the bores in the retainer for resisting play between the threads and/or increasing friction therebetween. an array of ball bearings for bearing against the orbiting scroll base in an axial direction, each ball bearing describing a circular path over the orbiting scroll base during orbiting motion; at least one thrust surface for bearing against the array of ball bearings; a retainer for retaining the ball bearings in location relative to said at least one thrust surface and the orbiting scroll; and an adjustment mechanism comprising a plurality of adjustment members angularly spaced apart about the axis of the pump operable for adjusting the axial position of said at least one thrust surface and thereby the axial position of the orbiting scroll base, wherein the adjustment members are threaded and co-operate with threaded bores in the retainer such that rotation of the adjustment members causes adjustment of the axial position and/or angle of the retainer; characterized in that the scroll pump further comprises a plurality of biasing members configured to bias the adjustment mechanism relative to the thrust surface or surfaces for resisting relative movement between the thrust surface or surfaces and the adjustment mechanism; said plurality of biasing members being for respective adjustment members for causing the threads of the adjustment members to bear against the threads of the bores in the retainer for resisting play between the threads and/or increasing friction therebetween.