We present the results of an interferometric study of 38 millimeter-wave lines of ^ { 12 } CH _ { 3 } OH in the vicinity of the massive star forming region W3 ( OH/H _ { 2 } O ) .
These lines cover a wide range of excitation energies and line strengths , allowing for a detailed study of excitation mechanisms and opacities .
In this paper we concentrate on the region around the water maser source W3 ( H _ { 2 } O ) and a region extending about 30 arcsec to the south and west of the hydroxyl maser source W3 ( OH ) .

The methanol emitting region around W3 ( H _ { 2 } O ) has an extent of 2.0 x 1.2 arcsec ( 4400 x 2600 AU ) .
The density is of order 10 ^ { 7 } cm ^ { -3 } , sufficient to thermalize most of the methanol lines .
The kinetic temperature is approximately 140 K and the methanol fractional abundance greater than 10 ^ { -6 } , indicative of a high degree of grain mantle evaporation .
The W3 ( H _ { 2 } O ) source contains sub-structure , with peaks corresponding to the TW source and Wyrowski ’ s B/C , separated by 2500 AU in projection .
The kinematics are consistent with these being distinct protostellar cores in a wide binary orbit and a dynamical mass for the region of a few tens of M _ { \sun } .

The extended methanol emission to the southwest of W3 ( OH ) is seen strongly only from the lowest excitation lines and from lines known elsewhere to be class I methanol masers , namely the 84.5 GHz 5 _ { -1 } –4 _ { 0 } E and 95.2 GHz 8 _ { 0 } –7 _ { 1 } A ^ { + } lines .
This suggests that this region , like class I maser sources , is dominated by collisional excitation .
Within this region there are two compact clumps , which we denote as swA and swB , each about 15 arcsec ( 0.16 pc projected distance ) away from W3 ( OH ) .
Excitation analysis of these clumps indicates the presence of lines with inverted populations but only weak amplification .
The sources swA and swB appear to have kinetic temperatures of order 50–100 K and densities of order 10 ^ { 5 } –10 ^ { 6 } cm ^ { -3 } .
The methanol fractional abundance for the warmer clump is of order 10 ^ { -7 } , suggestive of partial grain mantle evaporation .
The clumping occurs on mass scales of order 1 M _ { \sun } .