The ground state rotational spectrum of a hydrogen-bonded heterodimer formed from water and hydrogen cyanide has been detected and measured by using the technique of pulsed-nozzle, Fourier-transform microwave spectroscopy. Rotational constants (
B
0
,
C
0
) centrifugal distortion constants (
∆
J
,
∆
JK
) and, where appropriate,
14
N-, D- or
17
O-nuclear quadrupole coupling constants have been determined for the following isotopic species; H
2
16
O· · · HC
14
N, H
2
18
O· · · HC
14
N, H
2
16
O· · · HC
15
N, HD
16
O· · · HC
15
N, D
2
16
O· · · HC
15
N, H
2
16
O· · · DC
15
N, HD
16
O· · · DC
15
N and H
2
17
O· · · HC
15
N. An analysis of these spectroscopic constants indicates that the heterodimer is effectively planar, with a pair of equivalent protons and the arrangement H
2
O· · · HCN. The intermolecular interaction is through a hydrogen bond between HCN and H
2
O and the distance between the O and C nuclei
r
(O· · · C) is 3.157 Å (1Å = 10
-10
m). An interpretation of the nuclear quadrupole coupling constants leads to the conclusion that arccos <cos
2
Φ
>
½
≈ 51°, where
Φ
is the angle between the local C
2
axis of H
2
O and the
a
-axis of the complex; and that arccos <cos
2
θ
>
½
≈ 10°, where
θ
is the angle between the HCN axis and the
a
-axis. The intermolecular stretching force constant
k
σ
= 11 Nm
-1
has been determined from
∆
J
.
35Cl Nuclear Quadrupole Interaction in Thionyl Chloride