Infrared spectra of He-N-CO complexes with N up to about 20 have been observed in the 2145 cm-1 region of the C-O stretch vibration using a tunable diode laser spectrometer to probe pulsed supersonic expansions from moderately high pressure (less than or equal to40 atm) cooled (>-150 degreesC) jet sources. Cooler (less than or equal to0.2 K) or warmer (less than or equal to0.5 K) effective rotational temperatures were obtained using pinhole or slit jet nozzles, respectively. Two series of R(0) transitions were observed, each correlating smoothly with the known a-type (K=0<--0) and b-type (K=1<--0) R(0) lines of the binary complex, He-CO. Although the b-type series starts off about 7 times stronger for N=1, it was observed to lose intensity to the a series with increasing N-value. The numbering of cluster size was reliably established up to N=14 for the a-type and N=6 for the b-type series. Some warmer lines due to higher J-value transitions [e.g., R(1)] were observed and tentatively assigned, but these were not sufficient to enable rotational analysis. Thus it has not yet been possible to separate the effects of vibrational shifts and rotational dynamics on the line positions. Two critical regions were observed in the cluster size evolution around N=7 and 15, and these may be related to the theoretically calculated maximum and minimum, respectively, in the incremental binding energy per helium atom. (C) 2003 American Institute of Physics.