A recent paper in this journal points out that the ISO standards for LCA ignore the situation of internally recurring unit process in life cycle inventories. The paper also presents a way to solve this problem, using a set of simultaneous equations with one equation for each unit process and one variable for each product. The authors appear to be unaware of existing approaches towards this problem in which a set of simultaneous equations is used, but with a reverse set-up: one equation for each product and one variable for each unit process. This paper reviews the existing literature and contrasts it with the newly proposed method. It is concluded that the presentation of the new method is unclear in certain parts, and that its realm of application is probably restricted to product systems with mono-functional processes.