According to the houdini documention:

http://www.sidefx.com/docs/houdini/nodes/obj/geo [www.sidefx.com]

For correct matte shading of volumes:

Add the vm_matteshader property to the object.
Create a Volume Matte shader.
Set the density on this shader to match the density on the geometry shader.
Assign this shader to vm_matteshader.

Does any one have an example of a volume matte shader?

I want to hold out particle renders with a volume hold out matte.
i.e. where the density of the volume = 1, the particles are not visible in RGB and A.
Where the density of the volume = 0, the particles are visible in RGB and A.
And the particles are correspondingly partially visible for all values between 0 and 1.

I'm trying to make particles partially visible behind a translucent object.

Any pointers would be much appreciated.

thanks

just take any volume shader that you are using then
plug 0 to Cf and A
unplug F
keep Of as it is