Hello, mhsetup.sty (included by mathtools.sty) defines a few macros that expl3 wants to define anew, resulting in error messages. \documentclass{minimal} \usepackage{mathtools}% or mhsetup \usepackage{expl3}% error \begin{document} \null \end{document} This results in the following errors: % This is a LaTeX bug: check coding! % ! Command name `\if_num:w' already defined! Current meaning: \ifnum. % \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1} % % l.56 \cs_new_eq:NN \if_num:w % \tex_ifnum:D % This is a LaTeX bug: check coding! % ! Command name `\if_case:w' already defined! Current meaning: \ifcase. % \msg_kernel_bug:x ...oding!}\tex_errmessage:D {#1} % % l.57 \cs_new_eq:NN \if_case:w % \tex_ifcase:D % ! This is a LaTeX bug: check coding! % ! Command name `\if_dim:w' already defined! Current meaning: \ifdim % % For immediate help type H <return>. % ... % % l.169 \cs_new_eq:NN \if_dim:w % \tex_ifdim:D The macros \if_num:w, \if_dim:w, and \if_case:w are defined using \cs_new_eq:NN. Maybe you want to change this into \cs_set_eq:NN, or fix mhsetup.sty accordingly. Regards, Philipp