Parallel semi-continuous or continuous reactors are disclosed. The parallel reactors preferably comprise four or more reaction vessels. The reaction vessels are preferably small volume reaction vessels, preferably pressure reaction vessels, and/or preferably integral with a common reactor block. The reaction vessels can comprise shaft-driven stirrers. At least two, preferably at least three or at least four liquid feed lines can provide selective fluid communication between each of the reaction vessels and one or more liquid reagent sources. Additional features, suitable in connection with parallel reactors or with single reaction vessels are also disclosed.