Hi it seems that luatex (run with the plain format) ignores words inserted into exception dictionary which contain characters outside the ascii range. luatex -lua=opentype.lua -fmt=luatex hyphenation.tex hyphenation.tex --- \font\tenrm=lmroman10-regular.otf \tenrm \lccode "0104 = "0105 % Aogonek aogonek \lccode "0106 = "0107 % Cacute cacute \lccode "0118 = "0119 % Eogonek eogonek \lccode "0141 = "0142 % Lslash lslash \lccode "0143 = "0144 % Nacute nacute \lccode "00D3 = "00F3 % Oacute oacute \lccode "015A = "015B % Sacute sacute \lccode "0179 = "017A % Zacute zacute \lccode "017B = "017C % Zdotaccent zdotaccent \lefthyphenmin1 \righthyphenmin1 \hyphenation{a-b ó-ł} % oacute-lslash \showhyphens{ab ół} \end --- --Wlodek