Feeling like a total scrub now. Your correction works.
The particles are visible now and do turn (arround some wrong axis, though).
I think I can safely work from here. ^^
Thanks a lot, and sorry for this stupidity.
Edit:
Here is the new shader with a workarround using the View matrix for the billdboard orientation.
This way I don't need to use float3 at all.
[source lang="cpp"]VSOutput ParticleVS(VSInput input, InstanceData instance){ VSOutput output = (VSOutput)0;float lifeTimeFactor = (instance.T1 - T) / (instance.T1 - instance.T0);input.Position *= instance.Size * lifeTimeFactor;// transform to Screenspacefloat4x4 transform = transpose(View);transform[3] = 0;input.Position = mul(input.Position, transform); // Calculate the particle's position float dt = T - instance.T0; input.Position += instance.Position; input.Position += dt * instance.Velocity; input.Position += 0.5 * dt * dt * instance.Acceleration; float4 pos4 = mul(input.Position, World); pos4.w = 1; pos4 = mul(pos4, View); pos4 = mul(pos4, Projection); output.Position = pos4; output.TextureCoordinate = input.TextureCoordinate; output.Alpha = lifeTimeFactor; return output;}[/source]