An optical cell assembly for use in imaging the dissolution of a sample into a fluid stream, the assembly comprising: a hollow outer cell portion; and an insert portion shaped to releasably fit within the hollow outer cell portion; the insert portion defining a fluid flow channel extending in a direction along a length of the insert portion and which comprises an inlet, an outlet and a chamber disposed between the inlet and the outlet, the chamber having a maximum width dimension in a direction transverse to a main axis of the fluid flow channel which is greater than a corresponding width of the inlet; wherein the assembly further comprises: (i) a sample receiving area and a dissolution area comprised in the chamber, the sample receiving area being for supplying a sample to the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber; and/or (ii) a charging channel (68, 868, 368, 968) in the insert portion (14, 214) for supplying a sample into the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber, the charging channel (68, 868) being separate from the part of the fluid flow channel defining the inlet; characterised in that: the insert portion comprises opposed lateral windows (36, 38) which open on to the chamber on respective side surfaces (40, 42) of the insert portion, the chamber being closed when the insert portion is fitted within the outer cell portion; and the outer cell portion and the insert portion are arranged such that, when the insert portion is fitted within the outer cell portion, light from a light source can pass through the windows of the chamber to image dissolution or mixing of the sample into a fluid within the chamber. a hollow outer cell portion; and an insert portion shaped to releasably fit within the hollow outer cell portion; the insert portion defining a fluid flow channel extending in a direction along a length of the insert portion and which comprises an inlet, an outlet and a chamber disposed between the inlet and the outlet, the chamber having a maximum width dimension in a direction transverse to a main axis of the fluid flow channel which is greater than a corresponding width of the inlet; wherein the assembly further comprises: (i) a sample receiving area and a dissolution area comprised in the chamber, the sample receiving area being for supplying a sample to the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber; and/or (ii) a charging channel (68, 868, 368, 968) in the insert portion (14, 214) for supplying a sample into the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber, the charging channel (68, 868) being separate from the part of the fluid flow channel defining the inlet; (i) a sample receiving area and a dissolution area comprised in the chamber, the sample receiving area being for supplying a sample to the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber; and/or (ii) a charging channel (68, 868, 368, 968) in the insert portion (14, 214) for supplying a sample into the chamber such that the sample can dissolve in or mix into the fluid flowing through the chamber, the charging channel (68, 868) being separate from the part of the fluid flow channel defining the inlet; characterised in that: the insert portion comprises opposed lateral windows (36, 38) which open on to the chamber on respective side surfaces (40, 42) of the insert portion, the chamber being closed when the insert portion is fitted within the outer cell portion; and the outer cell portion and the insert portion are arranged such that, when the insert portion is fitted within the outer cell portion, light from a light source can pass through the windows of the chamber to image dissolution or mixing of the sample into a fluid within the chamber. the insert portion comprises opposed lateral windows (36, 38) which open on to the chamber on respective side surfaces (40, 42) of the insert portion, the chamber being closed when the insert portion is fitted within the outer cell portion; and the outer cell portion and the insert portion are arranged such that, when the insert portion is fitted within the outer cell portion, light from a light source can pass through the windows of the chamber to image dissolution or mixing of the sample into a fluid within the chamber.