Th is pape r describes a mathe ma ti cal a na lysis for de te rminin g the value of the substituti on e rror of a bolomete r with a Woll aston-w ire e leme nt (ba rrette r). The a na lys is re Aects a ll significant nonlin ea riti es in th e heat Aow, in c ludin g so me not co ve red be fore, a nd in cludes a ll apprec ia bl e heat trans port mec ha nisms simulta neously.. ' The va lues of s ub stituti o n e rror thu s obt a in e d , in co njun c tio n with e ffi cie ncy da ta obta me d b y mic rowave techniqu es, will be very useful in exte nding powe r me ter ca libra ti ons to freq ue ncy ra nges wh e re extre mely accura te mic rocalorime ters are not ava il a bl e.Key Word s : Barrette r, bolome te r, mi crowa ve powe r meas ure me nts, sub stituti on e rro r.