A combinatorial chemistry process in which material is vaporised from each of at least two sources (3,4) and deposited on a single substrate, wherein: the path of the vaporised material from each source (3,4) to the substrate is partially interrupted by an associated mask, wherein there is a single mask for a given source (3,4); the positioning of the mask in a plane parallel to the plane defined by the substrate is such that the material is deposited on the substrate in a thickness which increases substantially continuously in a direction along the substrate; a further plane is defined by the centre of the source (3,4) associated with that mask and the centre of the substrate, and intersecting an edge of the mask; the edge of the mask intersecting the further plane is within an area of the further plane defined by H 1, H 2, C 2 and C 1, wherein the area defined by H 1, H 2, C 2 and C 1 is the region within the quadrilateral area of the further plane defined by the points of the extremities of the substrate lying within the further plane (A 1,A 2) and the points of the extremities of the source lying within the further plane (C 1,C 2) that is at a perpendicular distance of greater than H y from the plane defined by the substrate, wherein H y is given by: H y = E + F A A + C wherein: E is the source to mask distance, said distance being defined as the perpendicular distance from the source (3,4) to the plane containing the mask; F is the mask to substrate distance, said distance being defined as the perpendicular distance from the plane containing the mask to the plane defined by the substrate; A is the substrate size, said substrate size being defined as the separation between the points of the extremities of the substrate lying within the further plane (A 1,A 2); and C is the source size, said source size being defined as the separation between the points of the extremities of the source lying within the further plane (C 1,C 2); and wherein each mask is moveable but is not moved in the course of the deposition process. the path of the vaporised material from each source (3,4) to the substrate is partially interrupted by an associated mask, wherein there is a single mask for a given source (3,4); the positioning of the mask in a plane parallel to the plane defined by the substrate is such that the material is deposited on the substrate in a thickness which increases substantially continuously in a direction along the substrate; a further plane is defined by the centre of the source (3,4) associated with that mask and the centre of the substrate, and intersecting an edge of the mask; the edge of the mask intersecting the further plane is within an area of the further plane defined by H 1, H 2, C 2 and C 1, wherein the area defined by H 1, H 2, C 2 and C 1 is the region within the quadrilateral area of the further plane defined by the points of the extremities of the substrate lying within the further plane (A 1,A 2) and the points of the extremities of the source lying within the further plane (C 1,C 2) that is at a perpendicular distance of greater than H y from the plane defined by the substrate, wherein H y is given by: H y = E + F A A + C wherein: E is the source to mask distance, said distance being defined as the perpendicular distance from the source (3,4) to the plane containing the mask; F is the mask to substrate distance, said distance being defined as the perpendicular distance from the plane containing the mask to the plane defined by the substrate; A is the substrate size, said substrate size being defined as the separation between the points of the extremities of the substrate lying within the further plane (A 1,A 2); and C is the source size, said source size being defined as the separation between the points of the extremities of the source lying within the further plane (C 1,C 2); and wherein each mask is moveable but is not moved in the course of the deposition process. E is the source to mask distance, said distance being defined as the perpendicular distance from the source (3,4) to the plane containing the mask; F is the mask to substrate distance, said distance being defined as the perpendicular distance from the plane containing the mask to the plane defined by the substrate; A is the substrate size, said substrate size being defined as the separation between the points of the extremities of the substrate lying within the further plane (A 1,A 2); and C is the source size, said source size being defined as the separation between the points of the extremities of the source lying within the further plane (C 1,C 2); and wherein each mask is moveable but is not moved in the course of the deposition process.