Combine the rhubarb, strawberries, sugar and water in a sauce pan and bring to a boil. Lower the heat and summer, stirring occasionally, until the fruit is soft and the liquid has thickened slightly. This will take 20-25 minutes. Allow to cool slightly.
Put a mesh strainer over a large bowl and pour the mixture through the strainer. Use a spoon to extract any extra liquid from the solids.
Carefully pour the syrup in a clean bottle, cover, and store in the refrigerator. (If you'd like, hang on to any leftover solid fruit pieces. They are almost like a jam and are delicious on toast.)