Methodology and apparatus for on-line UV monitoring of automated synthesis reactions. An apparatus includes a module with a fluidic cell the operational parameters of which remain substantially unchangeable not only during the process of monitoring but also in time between sequential processes. The module includes a separable housing structure containing a source of UV-light and an optical detector integrated with substantially temperature-insensitive fluidic cell. A portion of the cell is defined by a slot formed in a cell-chassis and complemented with inlet and outlet dimensioned to ensure that no air-bubble(s) and/or stagnating fluid is present in the cell during time when liquid reactants to-be-measured are delivered through the inlet into the cell. The method is configured to determine progression and completion of the reaction(s) and modification of reactions' times and repetitions of reaction(s) in real time.