A test set for stiff initial value problem solvers in the open source software R: Package deTestSet. We present the R package deTestSet that includes challenging test problems written as ordinary differential equations, differential algebraic equations of index up to 3 and implicit differential equations. In addition it includes 6 new codes to solve initial value problems. The R package is derived from the test set for initial value problem solvers available at url{ testset} which includes documentation of the test problems, experimental results from a number of proven solvers, and Fortran subroutines providing a common interface to the defining problem functions. Many of these facilities are now available in the R package deTestSet, which comprises an R interface to the test problems and to most of the Fortran solvers. The package deTestSet is free software which is distributed under the GNU general public license, as part of the R open source software project. (Source: