> \starttypescript[myFonts]
>     \definetypeface [mySerifFont] [rm] [serif] [mySerifFont] [default]
>     \definetypeface [mySansFont] [ss] [sans] [mySansFont] [default]
> \stoptypescript

\starttypescript[myFonts]
    \definetypeface [myFonts] [rm] [serif] [mySerifFont] [default]
    \definetypeface [myFonts] [ss] [sans] [mySansFont] [default]
\stoptypescript

You need the same name for both typefaces.

Are you sure ?

This works though...

\setuppapersize [A4][A4]

\starttypescript [serif] [mySerifFont]
    \definefontsynonym [mySerifFontRegular] [name:Times New Roman] [features=default]
    \definefontsynonym [mySerifFontItalic] [name:Times New Roman Italic] [features=default]
    \definefontsynonym [mySerifFontBold] [name:Times New Roman Bold] [features=default]
    \definefontsynonym [mySerifFontBoldItalic] [name:Times New Roman Bold Italic] [features=default]
\stoptypescript
\starttypescript [serif] [mySerifFont]
    \setups[font:fallback:serif]
    \definefontsynonym [Serif] [mySerifFontRegular]
    \definefontsynonym [SerifItalic] [mySerifFontItalic]
    \definefontsynonym [SerifBold] [mySerifFontBold]
    \definefontsynonym [SerifBoldItalic] [mySerifFontBoldItalic]
\stoptypescript

\starttypescript [sans] [mySansFont]
    \definefontsynonym [mySansFontRegular] [name:Arial] [features=default]
    \definefontsynonym [mySansFontItalic] [name:Arial Italic] [features=default]
    \definefontsynonym [mySansFontBold] [name:Arial Bold] [features=default]
    \definefontsynonym [mySansFontBoldItalic] [name:Arial Bold Italic] [features=default]
\stoptypescript
\starttypescript [sans] [mySansFont]
    \setups[font:fallback:sans]
    \definefontsynonym [Sans] [mySansFontRegular]
    \definefontsynonym [SansItalic] [mySansFontItalic]
    \definefontsynonym [SansBold] [mySansFontBold]
    \definefontsynonym [SansBoldItalic] [mySansFontBoldItalic]
\stoptypescript

\starttypescript[myFonts]
    \definetypeface [mySerifFont] [rm] [serif] [mySerifFont] [default]
    \definetypeface [mySansFont] [ss] [sans] [mySansFont] [default]
\stoptypescript

\usetypescript[myFonts]

\setupbodyfont[mySerifFont,12pt]

\definefont[myChapterFont][mySansFontRegular sa 1.728]

\setuphead[chapter][style=\mySansFont\myChapterFont]

\starttext

\chapter{chapter test}

\section{section test}

text test

\stoptext


Alan